The available end time was accidentally substracted by the start time
of the last period.
To avoid similar time reference confusion in the future, also renaming
many variables and methods to clearly reflect the time reference point.
And to avoid constant conversion, the processManifest method also
attempts to converge to time relative to the start of the window as
quickly as possible.
Issue: #8537
PiperOrigin-RevId: 357001624
d700990198