import datetime
import arcpy
arcpy.env.workspace = 'c:/temp/my_gdb.gdb'
fc = 'my_fc'
fld = 'my_date_field'
cur = arcpy.InsertCursor(fc)
row = cur.newRow()
row.setValue(fld, datetime.datetime.now())
cur.insertRow(row)
row = cur.newRow()
row.setValue(fld, datetime.datetime(2010,12,25,12,34)
cur.insertRow(row)
row = cur.newRow()
row.setValue(fld, datetime.datetime.now())
cur.insertRow(row)
a_date = '2010-12-25 13:57:01'
date_fmt = '%Y-%m-%d %H:%M:%S'
row = cur.newRow()
row.setValue(fld, datetime.datetime.strptime(a_date, date_fmt)
cur.insertRow(row)
del row
del cur
See python docs for the date and datetime modules for details. strptime and strftime to go between strings and datetime.