First week of January, 1981.
I think Pars had it mostly right.
The first shop you passed (before shop 29) was the first shop that sold the hot pasties. Also, on an odd year you came from the first shop that sells hot pasties so you came from the first shop (Jan).
I'm taking a wild guess that the number of the first shop is the actual day of the month you were born on. All we have is the cryptic if higher is lower and lower is higher bit. The shop you have stopped at is August (the current month as well as the opposite of Jan). I'm interpreting this to mean that the number of the first shop is actually lower (because higher is lower) then the current shop. The current shop number is 8 (August 8 is the posting date). So the number of the first shop is 1-7.
How convoluted can I get. Probably complete gibberish, but my brain really hurts now