The default terms of services will use "2016-06-06" as the date, it does not specify the time, and will thus use midnight in the time of the server. If the datebase is migrated from one time zone to another this will result in a different time stamp, resulting in all users having to accept the terms of services again.

Additionally the default terms of services will use this date, if the user adds a specific terms of services it will use the date+time of when it was submitted. If user accepts these new terms of services it will record that date time. Next the admin removes the terms of services, falling back to the default one, it will also reset the date to be that in the past, meaning all users will automatically accept the terms of services.

This is all around line 156 in AppConfigurationServices.