handle correctly addToDate when add/remove month

See merge request !2064
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Status Job ID Name Coverage
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passed SlapOS.Eggs.UnitTest-Master.Python2

00:17:13

passed SlapOS.Eggs.UnitTest-Master.Python3

00:35:30

passed Wendelin.UnitTest-Master

01:12:27

passed SlapOS.Eggs.UnitTest-Master.Python2

00:17:15

passed SlapOS.Eggs.UnitTest-Master.Python2

00:32:06

passed SlapOS.Eggs.UnitTest-Master.Python3

00:19:51

passed SlapOS.Eggs.UnitTest-Master.Python3

00:22:49