How many days in advance can I do my 90-day check-in in Thailand?

You can do your 90-day check-in in Thailand up to 15 days in advance. The reporting windows are as follows: in person at the local immigration office, you can check in from 15 days before until 7 days after the due date; online, you can report from 15 days before until the due date; and if using legacy mail (EMS), the post must be stamped at least 15 days before the due date.