DateTimeKey: don't depend on DateTime internals

use calendar module and public DateTime API instead.
6 jobs for feat/datetime3 in 0 seconds (queued for 1 second)
Status Job ID Name Coverage
  External
passed ERP5.CodingStyleTest-TestRunner1

01:41:13

passed ERP5.PerformanceTest-TestRunner1

00:29:17

failed ERP5.UnitTest-TestRunner1

01:39:08

passed ERP5.CodingStyleTest-TestRunner1

00:45:49

passed ERP5.PerformanceTest-TestRunner1

00:26:16

failed ERP5.UnitTest-TestRunner1

01:43:00