And rather giving the actual time, think of the clock in 12 hour increments. So, your starting point is AM (if giving insulin it would be AMPS - morning preshot), then when you test throughout the 12 hour cycle, it would be +1, +2, +3 up to +11.
Then you have PM (if giving insulin it would be PMPS - evening preshot).
Take a look at the spreadsheet template and hopefully that will help you understand it better.
You can find the how to set up the spreadsheet in the Tech Forum.
The reason we don't use the actual time of day (ie. 8:00, 10:30, etc) is because everyone is in different time zones and it's too hard to try to figure out what you did when.
Does that make sense?