Made Timezone format Outlook compatible

Summary:
According to e.g. https://docs.microsoft.com/en-us/windows/win32/api/minwinbase/ns-minwinbase-systemtime,
24 is not allowed in the Hour field, and consequently Outlook can't deal with it.
This is the same workaround that Outlook applies.

Thanks to alec for finding the issue.

Reviewers: #syncroton_developers, machniak

Subscribers: #syncroton_developers

Differential Revision: https://git.kolab.org/D2647