When BJ is talking about Erin, he says "I can't believe it's been almost a year..."
Earlier, in {"A War For All Seaons" (#9.6) } we see that 1950 makes place for 1951, which turns into 1952. So BJ has been in Korea for well over a year.
When Margaret is dancing with BJ, her fingernails can be seen to be more than 3/4 inch past the tips of her fingers. No nurse, especially a head nurse, would have fingernails that long.
It is highly unlikely that Hawkeye's anniversary surprise for BJ could have been planned and executed in the time allotted. Assuming that it is May 1st when BJ first mentions his anniversary that leaves 22 days for Hawkeye to make his recordings, mail them to the United States, Peg to have herself filmed, that film developed, and then mail it to Korea.