diff --git a/radicale/utils.py b/radicale/utils.py
index 1b5a016c..5c8d4100 100644
--- a/radicale/utils.py
+++ b/radicale/utils.py
@@ -17,6 +17,7 @@
 # You should have received a copy of the GNU General Public License
 # along with Radicale.  If not, see <http://www.gnu.org/licenses/>.
 
+import datetime
 import os
 import ssl
 import sys
@@ -46,6 +47,10 @@ ADDRESS_TYPE = Union[Tuple[Union[str, bytes, bytearray], int],
                      Tuple[str, int, int, int]]
 
 
+# Max YEAR in datetime in unixtime
+DATETIME_MAX_UNIXTIME: int = (datetime.MAXYEAR - 1970) * 365 * 24 * 60 * 60
+
+
 def load_plugin(internal_types: Sequence[str], module_name: str,
                 class_name: str, base_class: Type[_T_co],
                 configuration: "config.Configuration") -> _T_co:
@@ -244,3 +249,12 @@ def user_groups_as_string():
         username = os.getlogin()
         s = "user=%s" % (username)
     return s
+
+
+def format_ut(unixtime: int) -> str:
+    if unixtime < DATETIME_MAX_UNIXTIME:
+        dt = datetime.datetime.fromtimestamp(unixtime, datetime.UTC)
+        r = str(unixtime) + "(" + dt.strftime('%Y-%m-%dT%H:%M:%SZ') + ")"
+    else:
+        r = str(unixtime) + "(>MAX:" + str(DATETIME_MAX_UNIXTIME) + ")"
+    return r