My recommendation:
build it yourself!
Instead of timecode or date time, use the Numbers generator and a Ramp behavior.
You can make a 10 minute clock, minutes 0 - 9, and use plain text to enter a start time (you'll need to control opacity over the entire project in order to "cut in" to a time sequence that doesn't start on 0:00.)
You will need a seconds clock 0 - 60 (yes... 60! for 1 frame - you won't really see it, and if you can, it's like the number morphs from 5 to 0 quickly.)
Start a New Project. Length is up to you (90 minutes or 10 minutes if you follow my suggestion.)
It is IMPORTANT that the project be set for 30p (if you set it to 29.97 or slower, you will drive yourself nuts trying tweak the settings to be "dead on accurate" -- 60p will not work [for some reason... the generator "cuts out"])
Add a Numbers generator. Turn off Animate. Set Minimum Digits to 2.
Right click on Value and add a Parameter behavior : Ramp
Move the playhead to 00:01:00:00 (1 minute exactly) and set an out for the Number generator.
Set the Ramp End Value to 60.0.
Clone the Numbers Generator.
Go directly to Properties > Timing and set the end condition to Loop.
Drag out the clone layer to the end of the project. These are your seconds. They will be quite accurate save for 1 frame per cycle that will show "60" and immediately go to "00" (most people will never see it).
For Minutes: add another Number generator. Set minimum digits to 2 (if longer than 10 minutes). I made a project of 1 Hour, so I set the End Value to 59 (start value = 0 -- it defaults to 1). My guess is that a 10 minute project starting at 0 will require an end value of 9.
I added a "flashing" colon in between the minutes and seconds, plus a few other cosmetic effects (I used a Digital Display font that I created years ago and used copies of the numbers 88:88 in dark gray to serve as a base for the "lighted" numbers.)
I bundled this project up so you can take it apart and take a look at it; and put it here:
http://sight-creations.com/fxexchange/Digital_Clock_1Hr.zip
It should be easy to extrapolate how to make it run backwards 🙂
If you decide to add Hours, clone the 60 minute cycle of the second Number generator as was done with the seconds generator.
It's set up as an FCPX effect (but you can open the .motn filei from any location) - if you install it as one, create a Generator Category called "Time" or Timers, etc... I have included the font in the Media folder. I **believe** Motion (and FCPX, if installed) will find that font and use it without having to install it with your system fonts. If not... I guess it will need to be installed.
If you follow the setup instructions (30p...) this method is frame accurate throughout the play range; and if you play through it, you are not likely to see the "60" flash in one frame per cycle... Overall, it's pretty cool; and it's quite a bit more flexible than trying to adapt the other timing generators in Motion.
—HTH

PS, I did not go out of my way to make this effect for this thread, I'd already done work on it and have actually previously released a variation of it (Analog Clock) for FCPX...