[BNM] MySQL date time formats

[Oliver Marshall]

I am creating a web report pulling data from a MySQL database we have
here. 

The datetime field of this particular database (not created by me) is
set as the type "BIGINT" and the date and times in it are 13 number long
integers.

Is this a normal way of storing date/times in mysql? Any suggestions how
I can decipher the correct date from this ?

Olly