For tax purposes can I claim Jan 01 as my first day in the new state? Based on the "establishing residency" check-list, there are multiple dates between mid-December and the end of January that fit. If I were able to use Jan 01, that would certainly help with the complexity. Or do I need to use the first applicable date?
The primary question is when did your employer start withholding taxes for your new state?
If they started withholding after Jan. 1st (or you moved to a state that has no income taxes), then you can start the New Year as a state resident.
If they started withholding for the new state in December, you are going to have to file two state returns. At that point, you might set up your moving date on the day they withheld for the new state.
The two states are MD and PA. Since they have reciprocity, does that change anything? That said, they started withholding for PA after Jan 1. If it ends up where there’s carry over into 2020 I guess I’ll have to do the two state returns next year?
You should file a tax return for the states you were a resident of during the year, and/or the states that you had tax withheld. It is not real critical if you declare yourself a resident in December or January since you are dealing with reciprocal states, so it would seem best to choose that based on the ease of your tax preparation.