Check a programme around midnight
Take the date and time in the broadcaster schedule and compare it with the time zone attached to the feed. An event scheduled at 00:30 in London can appear on the previous evening for a viewer in North America. A current programme on both sides gives you an easier check than comparing weekday headings alone.
Look for an old cached schedule
If the channel’s schedule remains a full day behind around the clock, refresh the same guide source and look at the newest programme date it contains. Check that the player imported the right feed. Record the source’s date range and whether the problem affects one channel or all channels.
If the mapping is correct, leave the global offset in place and report the station, local date and time, and time zone to support. A one-channel error can come from the feed data even when the other stations show the right day.