Snapping a time parameter means changing that time parameter to include a wider range to ensure all the aggregate bars are full. We snap the ‘from’ parameter to the beginning of the resolution, and we snap the ‘to’ parameter to the end resolution. For example, minute bars begin at the top of the minute (9:30:00), hour bars begin at the top of the hour (9:00:00), daily bars begin at the beginning of the day (12:00:00 AM EST), weekly bars begin Sunday at 12:00:00 AM EST, monthly bars begin on the first day of the month and quarterly bars begin on the first day of the quarter. If you are trying to query the Open, High, Low, Close values from just the premarket times, you would need to use a ‘minute’ timespan and adjust the multiplier to 330 or 5.5 hours (4 AM - 9:30 AM). This would return one bar for the entire premarket. You can read more about this process in our blog post - Aggregate Updates