insert datetime based on a date field + fix time (12:00 noon)

I not sure how to do this. I'm creating a table that tells when a timesheets are due and this is a datetime field (timesheet_due). The timesheets are due at noon 5 days prior to the check date. The check date is from another table that I am using to create the timesheet table record. I can't get the syntax right to do this. insert into ts_table (keyfield, timesheet_due) select keyfield, ck_date - 5 units days ???something??? from pay_table where ..... What do I replace ???somthing??? to get 12:00 for the time? John