diff --git a/lib/actions/entry-actions.js b/lib/actions/entry-actions.js
--- a/lib/actions/entry-actions.js
+++ b/lib/actions/entry-actions.js
@@ -16,7 +16,10 @@
 } from '../shared/dm-ops/dm-op-utils.js';
 import { useProcessAndSendDMOperation } from '../shared/dm-ops/process-dm-ops.js';
 import { getNextLocalID } from '../shared/message-utils.js';
-import { type DMCreateEntryOperation } from '../types/dm-ops.js';
+import {
+  type DMCreateEntryOperation,
+  type DMEditEntryOperation,
+} from '../types/dm-ops.js';
 import type {
   RawEntryInfo,
   CalendarQuery,
@@ -34,7 +37,10 @@
 import type { HistoryRevisionInfo } from '../types/history-types.js';
 import type { ThreadInfo } from '../types/minimally-encoded-thread-permissions-types';
 import { thickThreadTypes } from '../types/thread-types-enum.js';
-import { dateFromString } from '../utils/date-utils.js';
+import {
+  dateFromString,
+  dateString as stringFromDate,
+} from '../utils/date-utils.js';
 import { useSelector } from '../utils/redux-utils.js';
 
 const fetchEntriesActionTypes = Object.freeze({
@@ -294,8 +300,80 @@
     };
   };
 
-function useSaveEntry(): (input: SaveEntryInfo) => Promise<SaveEntryResult> {
-  return useKeyserverCall(saveEntry);
+export type UseSaveEntryInput = $ReadOnly<
+  | {
+      +thick: false,
+      +saveEntryInfo: SaveEntryInfo,
+    }
+  | {
+      +thick: true,
+      +threadInfo: ThreadInfo,
+      +saveEntryInfo: SaveEntryInfo,
+    },
+>;
+function useSaveEntry(): (
+  input: UseSaveEntryInput,
+) => Promise<SaveEntryResult> {
+  const viewerID = useSelector(
+    state => state.currentUserInfo && state.currentUserInfo.id,
+  );
+
+  const processAndSendDMOperation = useProcessAndSendDMOperation();
+  const keyserverCall = useKeyserverCall(saveEntry);
+  const entryInfos = useSelector(state => state.entryStore.entryInfos);
+
+  return React.useCallback(
+    async (input: UseSaveEntryInput) => {
+      if (!input.thick) {
+        const result = await keyserverCall(input.saveEntryInfo);
+        return result;
+      }
+      const { saveEntryInfo, threadInfo } = input;
+      const prevEntry = entryInfos[saveEntryInfo.entryID];
+
+      invariant(viewerID, 'viewerID must be set');
+
+      const op: DMEditEntryOperation = {
+        type: 'edit_entry',
+        threadID: threadInfo.id,
+        creatorID: viewerID,
+        creationTime: prevEntry.creationTime,
+        lastUpdatedTime: saveEntryInfo.timestamp,
+        entryID: saveEntryInfo.entryID,
+        entryDate: stringFromDate(
+          prevEntry.year,
+          prevEntry.month,
+          prevEntry.day,
+        ),
+        text: saveEntryInfo.text,
+        messageID: uuid.v4(),
+      };
+      const opSpecification: OutboundDMOperationSpecification = {
+        type: dmOperationSpecificationTypes.OUTBOUND,
+        op,
+        recipients: {
+          type: 'all_thread_members',
+          threadID:
+            threadInfo.type === thickThreadTypes.THICK_SIDEBAR &&
+            threadInfo.parentThreadID
+              ? threadInfo.parentThreadID
+              : threadInfo.id,
+        },
+      };
+
+      void processAndSendDMOperation(opSpecification);
+
+      return {
+        entryID: saveEntryInfo.entryID,
+        newMessageInfos: [],
+        updatesResult: {
+          viewerUpdates: [],
+          userInfos: [],
+        },
+      };
+    },
+    [keyserverCall, processAndSendDMOperation, viewerID, entryInfos],
+  );
 }
 
 const deleteEntryActionTypes = Object.freeze({
diff --git a/native/calendar/entry.react.js b/native/calendar/entry.react.js
--- a/native/calendar/entry.react.js
+++ b/native/calendar/entry.react.js
@@ -26,6 +26,7 @@
   useDeleteEntry,
   useSaveEntry,
   type UseCreateEntryInput,
+  type UseSaveEntryInput,
 } from 'lib/actions/entry-actions.js';
 import { extractKeyserverIDFromIDOptional } from 'lib/keyserver-conn/keyserver-call-utils.js';
 import { registerFetchKey } from 'lib/reducers/loading-reducer.js';
@@ -38,7 +39,6 @@
   CreateEntryPayload,
   DeleteEntryInfo,
   DeleteEntryResult,
-  SaveEntryInfo,
   SaveEntryPayload,
   SaveEntryResult,
 } from 'lib/types/entry-types.js';
@@ -208,7 +208,7 @@
   +dispatchActionPromise: DispatchActionPromise,
   // async functions that hit server APIs
   +createEntry: (input: UseCreateEntryInput) => Promise<CreateEntryPayload>,
-  +saveEntry: (info: SaveEntryInfo) => Promise<SaveEntryResult>,
+  +saveEntry: (input: UseSaveEntryInput) => Promise<SaveEntryResult>,
   +deleteEntry: (info: DeleteEntryInfo) => Promise<DeleteEntryResult>,
   +canEditEntry: boolean,
 };
@@ -697,13 +697,26 @@
   ): Promise<SaveEntryPayload> {
     const curSaveAttempt = this.nextSaveAttemptIndex++;
     try {
-      const response = await this.props.saveEntry({
+      const saveEntryInfo = {
         entryID,
         text: newText,
         prevText: this.props.entryInfo.text,
         timestamp: Date.now(),
         calendarQuery: this.props.calendarQuery(),
-      });
+      };
+
+      const useSaveEntryInput = threadTypeIsThick(this.props.threadInfo.type)
+        ? {
+            thick: true,
+            threadInfo: this.props.threadInfo,
+            saveEntryInfo,
+          }
+        : {
+            thick: false,
+            saveEntryInfo,
+          };
+
+      const response = await this.props.saveEntry(useSaveEntryInput);
       if (curSaveAttempt + 1 === this.nextSaveAttemptIndex) {
         this.guardedSetState({ loadingStatus: 'inactive' });
       }
diff --git a/web/calendar/entry.react.js b/web/calendar/entry.react.js
--- a/web/calendar/entry.react.js
+++ b/web/calendar/entry.react.js
@@ -13,6 +13,7 @@
   useDeleteEntry,
   concurrentModificationResetActionType,
   type UseCreateEntryInput,
+  type UseSaveEntryInput,
 } from 'lib/actions/entry-actions.js';
 import {
   type PushModal,
@@ -26,7 +27,6 @@
 import { useThreadHasPermission } from 'lib/shared/thread-utils.js';
 import {
   type EntryInfo,
-  type SaveEntryInfo,
   type SaveEntryResult,
   type SaveEntryPayload,
   type CreateEntryPayload,
@@ -73,7 +73,7 @@
   +dispatch: Dispatch,
   +dispatchActionPromise: DispatchActionPromise,
   +createEntry: (input: UseCreateEntryInput) => Promise<CreateEntryPayload>,
-  +saveEntry: (info: SaveEntryInfo) => Promise<SaveEntryResult>,
+  +saveEntry: (input: UseSaveEntryInput) => Promise<SaveEntryResult>,
   +deleteEntry: (info: DeleteEntryInfo) => Promise<DeleteEntryResult>,
   +pushModal: PushModal,
   +popModal: () => void,
@@ -378,13 +378,26 @@
     const curSaveAttempt = this.nextSaveAttemptIndex++;
     this.guardedSetState({ loadingStatus: 'loading' });
     try {
-      const response = await this.props.saveEntry({
+      const saveEntryInfo = {
         entryID,
         text: newText,
         prevText: this.props.entryInfo.text,
         timestamp: Date.now(),
         calendarQuery: this.props.calendarQuery(),
-      });
+      };
+
+      const useSaveEntryInput = threadTypeIsThick(this.props.threadInfo.type)
+        ? {
+            thick: true,
+            threadInfo: this.props.threadInfo,
+            saveEntryInfo,
+          }
+        : {
+            thick: false,
+            saveEntryInfo,
+          };
+
+      const response = await this.props.saveEntry(useSaveEntryInput);
       if (curSaveAttempt + 1 === this.nextSaveAttemptIndex) {
         this.guardedSetState({ loadingStatus: 'inactive' });
       }