Yup.
If you have a cell phone whose time is regularly updated via your carrier its pretty much within 1-2 seconds of the nuclear powered Naval Observatory Master Clock.
I once hit the "Send IFT"button" about 10 seconds after it hit the top of the hour deadline, and the trade still went thru.
That seemed to indicate that maybe "30 seconds after the top of the hour" is the real cutoff.
You NEVER want to cut it that close, my issue was that the TSP.gov site timed out and I had to re log in. (lol).
A good tip on that:
If you've entered your IFT on the GOV site a few minutes before the deadline but want to watch "price" up intil the last minute, then go to your .Gov site and enter a space bar key back and forth, or re-type your IFT once every 2 minutes to keep the site from timing out (which is done for your security).