If you use a cursor to read in the data you can convert the values into a Python datetime object.That is much easier to round off and then write it out to a date field again.Note that you have to have a featureclass that handles datetime objects which does not include dBase and hence shapefiles.Note that rounding down is easy, just truncate the minutes and milliseconds.Rounding up is a bit trickier because the next minute may be a whole hour, that is where the datetime object makes it easy, we create a timedelta object of one minute that will do the tricky date arithmetic for us.In this case you want a floor function so no rounding is required.Python: batteries included.import datetime,math
exact = datetime.datetime.now()
newsecs = math.floor(exact.second/30.0)*30.0
newtime = exact.replace(second=newsecs,microsecond=0)
print exact,newtime
2011-08-17 21:37:46.296000 2011-08-17 21:37:30
2011-08-17 21:37:56.562000 2011-08-17 21:37:30
2011-08-17 21:38:02.171000 2011-08-17 21:38:00
2011-08-17 21:39:35.484000 2011-08-17 21:39:30
It would be possible to create a function that could be used in the field calculator. I don't recommend that. It is much easier to debug and add extra tests to use a cursor. What if the field is Null? In a cursor you can add more tests to trap unexpected data.
