Pick a number of times a week you'd like to eat out.
Multiply by fifty.
If you already celebrated your birthday this year, add 1753, otherwise add 1752.
Subtract your four digit birth year.
The first digit (or first two if you choose to eat out more than 9 times) is the number you started with.
The last two digits are your birthday.
The email claims that this only works this year, but it can work for the next millenium with this fix at line 5:
If you already celebrated your birthday this year, add (this_year - 2000) + 1750 otherwise (this_year - 2001) + 1750.
Or even better, it always works with (this_year - (this_year % 1000) * 1000)
Sorry, being a programmer.