compare these commands:
   $ date -u -d '31 mar 2001 utc + 86400 seconds'
   Sun Apr  1 00:00:00 UTC 2001

   $ date -u -d '1 apr 2001 utc + 86400 seconds'
   Mon Apr  2 01:00:00 UTC 2001

   $ date -u -d '2 apr 2001 utc + 86400 seconds'
   Tue Apr  3 00:00:00 UTC 2001

the second one is wrong by an hour.  any date calculation that
crosses some time in 1 apr 2001 is wrong by an hour.

another way this problem manifests, compare:
   $ date
   Sun Apr 15 02:12:03 PDT 2001

   $ date -d "1 jan 1970 utc + $(date +%s) seconds"
   Sun Apr 15 03:12:09 PDT 2001

the two should be almost the same, but the second one
is one hour in the future.