it depends on what your "number" is going to be;
SQL natively keeps the number of days since 01/01/1900:
select convert(decimal(12,0),getdate()) = 40167
select convert(decimal(12,4),getdate()) = 40166.5383
after the decimal point, it is the portion of one day(if 12:00 noon is 0,5, you can see i posted a bit later than that.
if your "number" is the number of seconds since a specific date, you'd have to tell us the starting date. how is your number going to represent time, if it is not a datetime column?
why not store the date no matter what? it's much more accurate and saves the whole convert from datetime to-number-back-todatetime problem.
does that help?
Lowell