I put the countdown on a calendar graphic.

If Feb 26 was T-21 days and today is T-18 days, then T-0 is Tuesday March 19.

In NASA shuttle countdowns (what we hear when counting 20, 19, 18 …) control of the clock is passed to the shuttle at T-31 seconds, there are no holds possible after T-31 (at least without cancelling the launch, or recycling way back) … when you get this far in the countdown you are about to fly.