90 days after 31st December 2017 is 31st March 2018. The earliest you should have been able to report would be 15 days before that (16th March), so Graham is correct, it is inconsistent.
I went to the labor office on the morning of my 50th birthday, cancelled my work permit, and then went to immigration in Khon Kaen and got an extension based on retirement. You need a bank letter (dated within 3 days in Khon Kaen), and the funds (800,000) must have been in your account for 2 months for your first extension (3 months thereafter).