My favourite way to time intervals and log times for each act is to use a Wait cue. Give it a sufficient length to have more than you expect to need, and then by starting it and pausing it at the right points, you effectively have a show timer running in the background. With the 'Active Cues' window visible, you can have visual countdown timers and stopwatches playing alongside your cues.
I usually run a pre-show sequence of wait cues that start automatically from the clock time, giving me an accurate count down with indicators as to when to give the half hour call, 5 min call, etc. I'll follow on into an 'overtime' counter which pauses when I play the first cue (usually fading house music), so I know from that whether we started 3 min late, for instance.
At the end of the show when all the cues have played I have paused timers left open, showing the times of each act, the interval time, etc.
Cheers,
Craig