we want to use dates for the calendarspec, and with that there are some
impossible combinations that cannot be detected during parsing
(e.g. some datetimes do not exist in some timezones, and the timezone
can change after setting the schedule)
so finding no timestamp is not an error anymore but a valid result
we omit logging in that case (since it is not an error anymore)
Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>