In practice, time zone offsets range from UTC-12:00 to UTC+14:00. Since each time zone has it's own concept of a day, if you want to cover the entire world you'll have to run your script until after 12:00 PM (Noon) UTC.
In other words, to cover any concept of May 1st, you'll have to wait until Noon UTC on May 2nd.
You might also want to a allow a few minutes for clock discrepencies. 12:05 PM UTC would work well.
HOWEVER - in many cases, you don't want to process the entire world at once. If you can separate the data by it's time zone, you may instead want to run a series of separate smaller batches after midnight in each time zone.