It's been inserting the current date and time, 1970, or all zero's depending on what I've tried. The current code inserts the current date and time.
Sorry it just occurred to me that you may need some additional info. To duplicate the issue.
Type an event name, pick a start date, and check the "All Day" checkbox.
Scroll down and check the "Online Checkin" checkbox
This should trigger the insert on line 196 to insert a record with NULL values for the checkin_open_date and checkin_close_date columns. The issue is that it inserts a value instead of NULL even when you leave the date and time fields on the form blank.
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