Harmonize format of start and end timestamp
Use TS_MW consistently because that's what we'll use for the local time.
Also make sure that all callers are passing in the right type.
This is preparatory work for the upcoming DST- and timezone-related
changes.
Bug: T315691
Bug: T311126
Change-Id: I56b575bd26228baf56dbc3116543a8fb242c61bd