diff --git a/bases/rsptx/admin_server_api/routers/instructor.py b/bases/rsptx/admin_server_api/routers/instructor.py
index 49ae9050c..ab0fb89b3 100644
--- a/bases/rsptx/admin_server_api/routers/instructor.py
+++ b/bases/rsptx/admin_server_api/routers/instructor.py
@@ -1206,15 +1206,18 @@ async def copy_assignment(
                 target_course=course,
                 importing_user=user,
             )
+            message = f'Copied as "{result.name}". It is hidden until you make it visible.'
+            if result.duedate_warning:
+                message += f" {result.duedate_warning}"
             return JSONResponse(
                 content={
                     "success": True,
-                    "message": f'Copied as "{result.name}". It is hidden until you '
-                    f"make it visible.",
+                    "message": message,
                     "imported": [result.name],
                     "skipped_existing": [],
                     "skipped_readings": result.skipped_readings,
                     "failed": [],
+                    "duedate_warning": result.duedate_warning,
                 }
             )
 
@@ -1237,16 +1240,25 @@ async def copy_assignment(
             parts.append(f"{len(bulk.failed)} could not be copied")
         if bulk.skipped_readings:
             parts.append(f"{bulk.skipped_readings} readings left behind")
+        if bulk.duedate_not_shifted:
+            parts.append(f"{bulk.duedate_not_shifted} due dates not adjusted")
+
+        message = ", ".join(parts) + ". Copies are hidden until you make them visible."
+        if bulk.duedate_not_shifted:
+            message += (
+                " Some due dates could not be adjusted because a course timezone "
+                "setting is invalid -- update it in Course Settings."
+            )
 
         return JSONResponse(
             content={
                 "success": True,
-                "message": ", ".join(parts)
-                + ". Copies are hidden until you make them visible.",
+                "message": message,
                 "imported": bulk.imported,
                 "skipped_existing": bulk.skipped_existing,
                 "skipped_readings": bulk.skipped_readings,
                 "failed": bulk.failed,
+                "duedate_not_shifted": bulk.duedate_not_shifted,
             }
         )
 
diff --git a/bases/rsptx/assignment_server_api/assignment_builder/src/components/routes/AssignmentBuilder/components/importAssignment/ImportAssignmentModal.spec.tsx b/bases/rsptx/assignment_server_api/assignment_builder/src/components/routes/AssignmentBuilder/components/importAssignment/ImportAssignmentModal.spec.tsx
index 1f428796c..72c34b14b 100644
--- a/bases/rsptx/assignment_server_api/assignment_builder/src/components/routes/AssignmentBuilder/components/importAssignment/ImportAssignmentModal.spec.tsx
+++ b/bases/rsptx/assignment_server_api/assignment_builder/src/components/routes/AssignmentBuilder/components/importAssignment/ImportAssignmentModal.spec.tsx
@@ -101,6 +101,7 @@ const {
         already_imported: false,
         imported_as: null,
         skipped_readings: 0,
+        duedate_warning: null,
         questions: [
           {
             id: 11,
diff --git a/bases/rsptx/assignment_server_api/assignment_builder/src/components/routes/AssignmentBuilder/components/importAssignment/ImportPreviewPanel.tsx b/bases/rsptx/assignment_server_api/assignment_builder/src/components/routes/AssignmentBuilder/components/importAssignment/ImportPreviewPanel.tsx
index 59596ffa0..e000d4c5f 100644
--- a/bases/rsptx/assignment_server_api/assignment_builder/src/components/routes/AssignmentBuilder/components/importAssignment/ImportPreviewPanel.tsx
+++ b/bases/rsptx/assignment_server_api/assignment_builder/src/components/routes/AssignmentBuilder/components/importAssignment/ImportPreviewPanel.tsx
@@ -68,6 +68,12 @@ export const ImportPreviewPanel = ({ assignmentId, onClose }: ImportPreviewPanel
             </Alert>
           ) : null}
 
+          {preview.duedate_warning ? (
+            <Alert variant="light" color="yellow" title="Due date not adjusted">
+              {preview.duedate_warning}
+            </Alert>
+          ) : null}
+
           {preview.skipped_readings > 0 ? (
             <Alert
               variant="light"
diff --git a/bases/rsptx/assignment_server_api/assignment_builder/src/store/assignment/assignment.logic.api.ts b/bases/rsptx/assignment_server_api/assignment_builder/src/store/assignment/assignment.logic.api.ts
index 207f37bbf..7de945b8c 100644
--- a/bases/rsptx/assignment_server_api/assignment_builder/src/store/assignment/assignment.logic.api.ts
+++ b/bases/rsptx/assignment_server_api/assignment_builder/src/store/assignment/assignment.logic.api.ts
@@ -39,6 +39,9 @@ export const ASSIGNMENT_TOAST_COPY = {
         } that belong to the other book. It is hidden until you make it visible.`
       : `Imported as "${name}". It is hidden until you make it visible.`,
   importError: "Couldn't import assignment. It may no longer be shared.",
+  // Shown separately from the success toast so it doesn't get lost inside a
+  // longer message -- the instructor needs to notice and go fix the timezone.
+  duedateWarning: (duedateWarning: string) => duedateWarning,
   loadTreeError: "Couldn't load shared assignments. Try again.",
   // Partial success is a normal outcome for a multi-import, so the toast counts
   // rather than claiming everything worked.
@@ -47,6 +50,7 @@ export const ASSIGNMENT_TOAST_COPY = {
     skipped_existing: string[];
     skipped_readings: number;
     failed: string[];
+    duedate_not_shifted: number;
   }) => {
     const parts = [`Imported ${result.imported.length}`];
 
@@ -59,6 +63,9 @@ export const ASSIGNMENT_TOAST_COPY = {
     if (result.skipped_readings) {
       parts.push(`${result.skipped_readings} readings left behind`);
     }
+    if (result.duedate_not_shifted) {
+      parts.push(`${result.duedate_not_shifted} due dates not adjusted`);
+    }
     return `${parts.join(", ")}. Imports are hidden until you make them visible.`;
   }
 } as const;
@@ -260,6 +267,15 @@ export const assignmentApi = createApi({
         queryFulfilled
           .then(({ data }) => {
             notify.success(ASSIGNMENT_TOAST_COPY.importedMany(data.detail));
+            if (data.detail.duedate_not_shifted) {
+              notify.info(
+                ASSIGNMENT_TOAST_COPY.duedateWarning(
+                  "Some due dates could not be adjusted because a course timezone " +
+                    "setting is invalid. Update it in Course Settings, then check " +
+                    "the affected assignments."
+                )
+              );
+            }
           })
           .catch(() => {
             notify.error(ASSIGNMENT_TOAST_COPY.importError);
@@ -279,6 +295,9 @@ export const assignmentApi = createApi({
             notify.success(
               ASSIGNMENT_TOAST_COPY.imported(data.detail.name, data.detail.skipped_readings)
             );
+            if (data.detail.duedate_warning) {
+              notify.info(ASSIGNMENT_TOAST_COPY.duedateWarning(data.detail.duedate_warning));
+            }
           })
           .catch(() => {
             notify.error(ASSIGNMENT_TOAST_COPY.importError);
diff --git a/bases/rsptx/assignment_server_api/assignment_builder/src/types/assignmentSharing.ts b/bases/rsptx/assignment_server_api/assignment_builder/src/types/assignmentSharing.ts
index 54b0bdddb..4df9cf089 100644
--- a/bases/rsptx/assignment_server_api/assignment_builder/src/types/assignmentSharing.ts
+++ b/bases/rsptx/assignment_server_api/assignment_builder/src/types/assignmentSharing.ts
@@ -36,6 +36,8 @@ export type SharedAssignmentPreview = {
   imported_as: string | null;
   /** How many readings will be left behind, which only happens cross-book. */
   skipped_readings: number;
+  /** Set when the due date could not be shifted because a course's timezone is invalid. */
+  duedate_warning: string | null;
   questions: SharedAssignmentQuestion[];
 };
 
@@ -44,6 +46,8 @@ export type ImportAssignmentResult = {
   id: number;
   name: string;
   skipped_readings: number;
+  /** Set when the due date could not be shifted because a course's timezone is invalid. */
+  duedate_warning: string | null;
 };
 
 /** One assignment under a course in the import tree. */
@@ -103,4 +107,6 @@ export type ImportCourseResult = {
   skipped_existing: string[];
   skipped_readings: number;
   failed: string[];
+  /** How many imports kept their original due date because a course timezone is invalid. */
+  duedate_not_shifted: number;
 };
diff --git a/bases/rsptx/assignment_server_api/routers/instructor.py b/bases/rsptx/assignment_server_api/routers/instructor.py
index 6e9c08eb4..83cbfeb34 100644
--- a/bases/rsptx/assignment_server_api/routers/instructor.py
+++ b/bases/rsptx/assignment_server_api/routers/instructor.py
@@ -2235,6 +2235,7 @@ async def import_assignment_endpoint(
             "id": result.assignment.id,
             "name": result.name,
             "skipped_readings": result.skipped_readings,
+            "duedate_warning": result.duedate_warning,
         },
     )
 
@@ -2374,6 +2375,7 @@ async def import_course_assignments_endpoint(
             "skipped_existing": result.skipped_existing,
             "skipped_readings": result.skipped_readings,
             "failed": result.failed,
+            "duedate_not_shifted": result.duedate_not_shifted,
         },
     )
 
diff --git a/components/rsptx/db/crud/assignment.py b/components/rsptx/db/crud/assignment.py
index 20fb5fb16..058d4fd4c 100644
--- a/components/rsptx/db/crud/assignment.py
+++ b/components/rsptx/db/crud/assignment.py
@@ -1,6 +1,6 @@
 import datetime
 from typing import NamedTuple, Optional, List
-from zoneinfo import ZoneInfo
+from zoneinfo import ZoneInfo, ZoneInfoNotFoundError
 from fastapi import HTTPException, status
 from asyncpg.exceptions import UniqueViolationError
 from rsptx.validation import schemas
@@ -1504,24 +1504,44 @@ def term_start_utc(
 
 def shift_duedate_between_courses(
     duedate: datetime.datetime, source_course, target_course
-) -> datetime.datetime:
+) -> tuple[datetime.datetime, Optional[str]]:
     """Re-date ``duedate`` so it keeps its offset from the start of term.
 
     Both term starts are anchored in their own course timezone, so the offset
     is preserved as local wall clock time even when the two terms fall on
     opposite sides of a DST change. If either course is missing a term start
     there is nothing to shift against, so the original date is kept.
+
+    A course timezone that is no longer a recognized zoneinfo key (a user can
+    pick one that later gets removed from the tz database) would otherwise
+    take the whole import down with it. That is worse than an unshifted due
+    date, so the shift is skipped and a warning string is returned instead of
+    raising -- the caller still gets an assignment, just not a re-dated one.
     """
     if not (source_course.term_start_date and target_course.term_start_date):
-        return duedate
+        return duedate, None
 
-    due_delta = duedate - term_start_utc(
-        source_course.term_start_date, source_course.timezone
-    )
-    return (
-        term_start_utc(target_course.term_start_date, target_course.timezone)
-        + due_delta
-    )
+    try:
+        due_delta = duedate - term_start_utc(
+            source_course.term_start_date, source_course.timezone
+        )
+        shifted = (
+            term_start_utc(target_course.term_start_date, target_course.timezone)
+            + due_delta
+        )
+    except (ZoneInfoNotFoundError, ValueError) as e:
+        rslogger.warning(
+            f"Could not shift due date from {source_course.course_name!r} to "
+            f"{target_course.course_name!r}, one of their timezones is invalid "
+            f"({source_course.timezone!r} -> {target_course.timezone!r}): {e}"
+        )
+        return duedate, (
+            "The due date could not be adjusted because a course timezone "
+            "setting is invalid. Update the timezone in Course Settings, then "
+            "check this assignment's due date."
+        )
+
+    return shifted, None
 
 
 def _normalize_assignment_kind(kind: str, is_timed: bool, is_peer: bool):
@@ -1579,12 +1599,16 @@ class ImportedAssignment(NamedTuple):
 
     ``skipped_readings`` is how many rows were left behind by the cross-book
     rule above, so the caller can say so rather than letting the instructor
-    discover the gap themselves.
+    discover the gap themselves. ``duedate_warning`` is set when the due date
+    could not be shifted to the target course's term because a course
+    timezone is invalid -- the assignment still imports, with its original
+    due date.
     """
 
     assignment: AssignmentValidator
     name: str
     skipped_readings: int
+    duedate_warning: Optional[str] = None
 
 
 def _searchable_column(field: str, joined_columns: dict):
@@ -2029,8 +2053,11 @@ async def fetch_assignment_for_preview(
         )
 
     duedate = assignment.duedate
+    duedate_warning = None
     if target_course is not None:
-        duedate = shift_duedate_between_courses(duedate, source_course, target_course)
+        duedate, duedate_warning = shift_duedate_between_courses(
+            duedate, source_course, target_course
+        )
 
     cross_book = (
         target_course is not None
@@ -2066,6 +2093,7 @@ async def fetch_assignment_for_preview(
         "already_imported": source_assignment_id in existing_imports,
         "imported_as": existing_imports.get(source_assignment_id),
         "skipped_readings": will_import.count(False),
+        "duedate_warning": duedate_warning,
         "questions": [
             {
                 "id": row.Question.id,
@@ -2109,7 +2137,7 @@ async def import_assignment(
         source_assignment_id, importing_user.id
     )
 
-    duedate = shift_duedate_between_courses(
+    duedate, duedate_warning = shift_duedate_between_courses(
         assignment.duedate, source_course, target_course
     )
     is_timed, is_peer = _normalize_assignment_kind(
@@ -2209,7 +2237,7 @@ async def import_assignment(
         f"into {target_course.course_name} as {result.id} ({new_name}) with "
         f"{len(rows_to_import)} questions, {skipped_readings} readings skipped"
     )
-    return ImportedAssignment(result, new_name, skipped_readings)
+    return ImportedAssignment(result, new_name, skipped_readings, duedate_warning)
 
 
 class BulkImportResult(NamedTuple):
@@ -2218,12 +2246,16 @@ class BulkImportResult(NamedTuple):
     Names rather than counts so a caller can show either. ``skipped_existing``
     holds the source names that were passed over, which is the difference
     between "that course only had two new ones" and "nothing happened".
+    ``duedate_not_shifted`` counts imports whose due date could not be moved
+    to the target term because a course timezone is invalid -- the assignment
+    still imports, just with its original due date.
     """
 
     imported: List[str]
     skipped_existing: List[str]
     skipped_readings: int
     failed: List[str]
+    duedate_not_shifted: int = 0
 
 
 async def import_course_assignments(
@@ -2324,6 +2356,7 @@ async def import_course_assignments(
     skipped_existing: List[str] = []
     failed: List[str] = []
     skipped_readings = 0
+    duedate_not_shifted = 0
 
     for source in rows:
         if skip_existing and source.id in already_imported:
@@ -2343,13 +2376,18 @@ async def import_course_assignments(
             continue
         imported.append(result.name)
         skipped_readings += result.skipped_readings
+        if result.duedate_warning:
+            duedate_not_shifted += 1
 
     rslogger.info(
         f"Bulk imported {len(imported)} assignments from {source_course.course_name} "
         f"into {target_course.course_name}; {len(skipped_existing)} already present, "
-        f"{len(failed)} failed, {skipped_readings} readings skipped"
+        f"{len(failed)} failed, {skipped_readings} readings skipped, "
+        f"{duedate_not_shifted} due dates not shifted"
+    )
+    return BulkImportResult(
+        imported, skipped_existing, skipped_readings, failed, duedate_not_shifted
     )
-    return BulkImportResult(imported, skipped_existing, skipped_readings, failed)
 
 
 async def search_shareable_courses(
diff --git a/test/components/rsptx/db/test_assignment_sharing.py b/test/components/rsptx/db/test_assignment_sharing.py
index 20656e1e6..f49015d67 100644
--- a/test/components/rsptx/db/test_assignment_sharing.py
+++ b/test/components/rsptx/db/test_assignment_sharing.py
@@ -643,6 +643,42 @@ async def test_import_shifts_the_due_date_by_the_offset_from_term_start(sharing_
     )
 
 
+async def test_import_keeps_the_original_duedate_when_a_course_timezone_is_invalid(
+    sharing_world,
+):
+    """A timezone the tz database no longer recognizes should not sink the import.
+
+    An instructor can end up with a course timezone that zoneinfo later stops
+    recognizing (a deprecated alias dropped from the tz database). The due
+    date shift depends on being able to resolve that zone, so it is skipped
+    rather than raising -- the assignment still imports, just without a
+    re-dated due date, and the caller gets a warning to relay.
+    """
+    bad_tz_course = await _make_course(
+        "sharing_bad_tz_course",
+        SRC_BOOK,
+        datetime.date(2026, 1, 12),
+        "Not/A_Real_Zone",
+    )
+    await create_instructor_course_entry(sharing_world["owner"].id, bad_tz_course.id)
+    await _share_assignments(bad_tz_course.id)
+
+    stored = datetime.datetime(2026, 1, 20, 23, 59)
+    source_assignment = await _make_assignment(
+        bad_tz_course.id, "Bad Timezone Homework", is_private=False, duedate=stored
+    )
+
+    result = await import_assignment(
+        source_assignment_id=source_assignment.id,
+        target_course=sharing_world["dst"],
+        importing_user=sharing_world["importer"],
+    )
+
+    assert result.assignment.duedate == stored
+    assert result.duedate_warning is not None
+    assert "timezone" in result.duedate_warning.lower()
+
+
 # Already imported
 # ----------------