Booking a round trip to Thailand

It really depends on the flight and the difference in timezone. Depending where you are in the US, the difference can change. Last time I went to Thailand from Canada, I was in the same timezone as NYC and left on a Thursday to arrive on Saturday afternoon. So if I had to work on the 1st of December, the flight would have to be on the 28th, arriving late on the 30th and therefore being able get started work on the 1st of December.

The way back is simpler. You are working Dec 1 to 15th, so you either have to take a very late flight on the 15th or book one for the 16th. Remember, it takes a while to get to the airport and it takes time to go through checking, immigration, security and boarding. So, your flight must be late enough to allow for all that after you are done what you have to do there.

There is no single answer, there are multiple possibilities. If you catch an early morning flight heading west you could reach Bangkok late at night the next day. If you catch a flight later in the day, you may arrive on the 3rd day. There are no flights that would leave and arrive the same day going to Bangkok.

Best bet is to simply try a flight search on the 30th, then see which flights arrive the next day and which arrive the 3rd day. Then try the day before and decide which works best price and itinerary wise.

For the return journey, it will depend on when you actually finish at the end of your "two week stint". Flights depart all day long from Bangkok, so you need to ask your job organizer when you actually finish your stint and can depart.

This also partially depends on where in the US you are.

1. There are no non-stop flights so you have to connect someplace
2. Typical connection would be Europe, Middle East, Japan or China.
3. Total flight time from most US locations will be around 24 hours
4. Time difference is 12 hours from the east coast, 15 hours from the west coast
5. So you will likely arrive around 40 hours after departure (both in local time)
6. There are search engines, where you can specify arrival time & date instead of departure. See for example https://matrix.itasoftware.com/