Both "time since last wind update" and "windgrid timestamp + a time zone"
are equivalent, if configured right.
Look at the "Forecast Age" clock (in the track designer panel).
It should show 00:00:01 after a wind shift time and
11:59:59 before the next wind shift, when it resets to 0 again.
The forecast age is used by the track designer and the weather routing module,
so it is important to set it correctly.