Group: " - ABC GROUP - "

Perhaps I am missing something, but I am confused about when the letter of the day changes to the next letter. We are all in different time zones and as far as I know, I cannot tell at what time the posting of photos under a new letter started. Therefore, I cannot tell if 24 hours have passed since use of the present letter began and if it may be time to change to the next letter. So, how do we know that 24 hours have passed and the letter should be changed. How is that decision made and who makes it?