diff --git a/cli/onionshare_cli/web/share_mode.py b/cli/onionshare_cli/web/share_mode.py
index 1dd9af13..8ac4055e 100644
--- a/cli/onionshare_cli/web/share_mode.py
+++ b/cli/onionshare_cli/web/share_mode.py
@@ -25,7 +25,7 @@ import sys
 import tempfile
 import zipfile
 import mimetypes
-from datetime import datetime
+from datetime import datetime, timezone
 from flask import Response, request, render_template, make_response, abort
 from unidecode import unidecode
 from werkzeug.http import parse_date, http_date
@@ -127,7 +127,7 @@ class ShareModeWeb(SendBaseModeWeb):
 
         self.download_etag = None
         self.gzip_etag = None
-        self.last_modified = datetime.utcnow()
+        self.last_modified = datetime.now(tz=timezone.utc)
 
     def define_routes(self):
         """
@@ -285,6 +285,8 @@ class ShareModeWeb(SendBaseModeWeb):
         if_unmod = request.headers.get("If-Unmodified-Since")
         if if_unmod:
             if_date = parse_date(if_unmod)
+            if if_date and not if_date.tzinfo:
+                if_date = if_date.replace(tzinfo=timezone.utc) # Compatible with Flask < 2.0.0
             if if_date and if_date > last_modified:
                 abort(412)
             elif range_header is None: