Date string in automatic snapshot has colon instead of plus

I just noticed that the auto snapshots that are being created on this server have the format
auto-2022-09-09T0000:0200
but the naming schema i provided does not contain a colon.
This is the format string:
%Y-%m-%dT%H%M%z
%z should be replaced with
+0200
as it does when i run

anyone know where the colon comes from? i checked strftime(3) but could not find any reason why there should be a colon.