Yes I have changed the off-shift time through 'Change Working Time'.
My shift times are also accurate because my shift times either end or start on the hour or at half past the hour. The 15min interval would always fall on these times. When i zoom out to hourly intervals the shifts finsihing at half past are shown to round up to the next hour.
Are you changing the shift times so that if for example the shift ended at 00:08 and your timescale is in 15min intervals, the off shift shaded area would finish actually at 00:08 and not 00:15?