maybe on latest revs of the 624 and most likely on the 824
with the tva

I just saw your question, so in case you still have not solved this, here is the answer.  I set the TVS50 for PBX mode, not automatic mode and did not use the time settings.  I set the time settings for day night through the 624.  I set 4 flex buttons on the main phone (x101) as follows,  

782#, store, - this is for night mode
781#, store, - this is for day mode
783#, store, - this is for lunch mode
780#, store, this is for cancel mode.

If you are in day mode, just hit the intercom and the flex button for night mode, and you are now in night mode.  Technically, when you come back in the morning at the next scheduled day mode setting, it should be back in day mode, though I have found this to be not totally reliable.  Also, if in day mode, change to night mode, then come back and hit cancel mode, it sometimes stays in night mode.  So I try not to use the cancel mode option anymore.  

Hope this helps..

Doc