I've always just said it letter by letter - D-A-T-Y. Grammatically, it could probably go either way on pronunciation. Since Y is sometimes used as a vowel, the general rule is when a vowel is followed by a consonant then another vowel, the first vowel is a long vowel, so it would be "day tee." However, since Y is a consonant it could be pronounced "dat tee." I'd say "day tee" is probably correct, but this link -
http://www.reference.com/motif/society/what-is-daty - has the pronunciation as "dah-tee."
So, say it however you'd like.
And we do have a member here on ECCIE - DATYMAN -
http://www.eccie.net/member.php?u=685