TDY’s are for the entire bid month.
They positive-space you there at the beginning & home at the end. Anything in-between is on you.
There are some general seasonal trends of how they TDY, but it’s hard to predict. You won’t know if your base is affected ‘till bids come out the month prior. You might never get hit some pilots have been TDY’d 6 or more months in a year. I personally know 3 pilots that I flew with who quit & moved on because of repeated TDY.