To calculate 16 days for filing, count backwards, with "1" being the day before. So, counting backwards from 9/15/10, 8/23/10 was the date for filing.
If you serve it by mail, you need to subtract 5 days backwards -- it should have been served on 8/18/10. If you serve in person, you could have served today. If you serve by overnight mail, it is 2 days you count backwards, so, you should have served by 8/21/10. The judge looks at the method of service, not the actual "effect" of service.
Hope that helps!
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You missed the deadline if you did not file it on Monday, Aug. 23 for a hearing on Sept. 15. Thus, the clerk may reject the motion or may simply mark it late or "received." If the date is fixed for some reason, then you should try to file your motion in any event. The mailing adds five calendar days to the required notice, but you can overnight it so it is then just two court days.