diff --git a/radicale/utils.py b/radicale/utils.py
index ed6c4ab2..f70e38e7 100644
--- a/radicale/utils.py
+++ b/radicale/utils.py
@@ -280,10 +280,7 @@ def format_ut(unixtime: int) -> str:
         # TODO check how to support this better
         return str(unixtime)
     if unixtime < DATETIME_MAX_UNIXTIME:
-        if sys.version_info < (3, 11):
-            dt = datetime.datetime.utcfromtimestamp(unixtime)
-        else:
-            dt = datetime.datetime.fromtimestamp(unixtime, datetime.UTC)
+        dt = datetime.datetime(1970, 1, 1, tzinfo=datetime.timezone.utc) + datetime.timedelta(seconds=unixtime)
         r = str(unixtime) + "(" + dt.strftime('%Y-%m-%dT%H:%M:%SZ') + ")"
     else:
         r = str(unixtime) + "(>MAX:" + str(DATETIME_MAX_UNIXTIME) + ")"