Can I apply for a 1-year extension for my children's 90-day non-ED visa up to the expiry date?

With our kids 90 day non-ED visa, do we have up until its expiry to apply for the 1 year extension? (noting that we're still waiting for school paperwork to be completed & their visas expire 4th Feb). I've tried to research this myself, but all that I can find that is definitive is that you can apply after 45 days have elapsed from the issue date, but then conflicting info as to what's the last day you can apply for the renewal ... with 15 & 30 days often cited, as well as, right up until the expiry date. Could someone please clarify for me? Thanks!