For a timer kind of thing like that I would usually do the lazy approach and have an excel document do some level of automation with it.
Like =360-(TODAY()-(STARTINGDAY)) = (REDUCED TIME) //Wouldn't hurt to have these values in plain sight either.
So, for example: On Day-0, 360 - (1/1/15 - 1/1/15) = 360 or 360 - (1/5/15 - 1/15/15) = 350
In another cell, you can also =CONVERT((REDUCED TIME),SECOND,MINUTE) to reduce the hassle of calculating it. And in other cells, you can set benchmarks from there (10 push-ups to 50 * (REDUCED TIME)), and even have a converter for how long you want to spend, and have it auto-calculate how many you'll need to do to reach it.
I've done something somewhat similar with my freelancing; giving myself benchmarks as to how much I would feel comfortable working to earn both time and money. Considering my overall laziness; I won't want to work all the time, so I'll calculate how much I would feel comfortable earning as to not worry anymore (minimum being enough to pay the bills and to eat), and do the rest from there. Actually makes time management more fun; provided I don't waste all that time adding onto the calculator.
EDIT:
Corrected a miscalculaton.
EDIT EDIT:
Nevermind. I had it right before.
EDIT EDIT EDIT:
Additional happy, since I mentioned my freelancing. I think I have a monthly recurring client now. HUZZAH! Not the most generous payout, relatively speaking, but every little bit stacks up. Plus, I'm not needed for that job for long anyhow, and given the time it costs (by the amount I charge per-hour), the payout is actually worth it. Plus, it's a simple enough job that I don't need to fry my brain working on it (also fitting within the tight timeframe is pretty easy). Additionally, the client's actually pretty cool, and helpful and patient. More like this would be appreciated.
EDIT EDIT EDIT EDIT:
Just got an invite for another job that sounds somewhat similar. Neat. Now let's hope I beat out everyone else applying for it.