This is the sort of thing you'd learn from your dba or vendor.
If you had an idea of what the result should be, we might figure it out.
Generally it's the number of seconds from some point in time, but without knowing that date one can only guess.
The length of this value makes me nervous too, perhaps it's milliseconds, but check with the developer/vendor.
The formula might be something like:
dateadd("s",(127264599743590905/1000),cdate(1970,1,1))
-k