time: add portable wrapper struct for tm
time: add portable wrapper struct for tm

This wrapper struct adds an explicit time zone offset field on
platforms that do not include such a field within `struct tm`,
such as on Windows. This gives us a place to store parsed
time zone information so that we don't have to drop it.

Signed-off-by: Jeff Erbrecht <4f7292804c97411a4a82ba562509268e2747b6b8@google.com>

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