Every time you start to do something, simply enter a TimeTweet (TM) containing #hashtags.
Don't worry about the starting and ending times - the task starts when you enter the TimeTweet and continues until your next TimeTweet. The 'clock' is always running, so make sure you enter a TimeTweet like '#Untracked' when you want to go 'off the clock'. (Don't worry, you can revise your TimeTweets if you have to!)
Each TimeTweet can have a #hashtag, just like Twitter. TimeStitcher will show you how long you spend on each #hashtag.