The "suggested" dress code for My Time Dining is the same as the main dining room.
On non-formal nights, the suggested dress is smart casual, which, for men, means Dockers-type pants, and a golf shirt. For ladies, it's probably slacks and a nice top (ladies, help me out here).
On formal nights, it's a suit/tux for men, formal gown/pants suit for ladies.
Shorts are not permitted at dinner in any venue (except, maybe, the Windjammer buffet).
Having said all of the above, I can tell you that you will probably see guys in jeans and a ball cap trying to go to dinner in the MDR on formal night. Depending on the ship, and the personnel at the door, they may or may not get in.
This is one of my main complaints, as well as others who will probably post. If the cruise line wants to have men in tuxedos or suits, then they should remove the word "suggested" from the dress code. Same applies to women.
We have seen everything from very formal dress to VERY casual dress in the MDR on formal night.
This subject probably gets more comments than any other, except, maybe, the subject of "chair reserving" at the pools.
Whatever you decide to do, enjoy your cruise.