Lots and lots of folks make the mistake of assuming the following
- 90 days means 3 months before! It does NOT! If you were approved for GC on 7/31, 90 days before is 4/30 +1 = 5/1 since May has 31 days.
- some people think - " oh well I can mail the application 3-4 days early as when they receive it it will be 90 days before. This is a very dangerous game. There are no clear guidelines on whether the 90 days before is the date you sign or the date they receive the mail.
Always give some slack. Give at least 1 week slack after you become eligible to avoid any confusion.
In UK citizenship form, there is one condition that 5 yrs before you submit, you should have been present in the country.
According to their website, most of the people FAIL the eligibility requirement for this very reason - they do not pay attention to when they turn in their form they do not assess where they were EXACTLY 5 yrs before.
Ok, the above requirement does not hold for US Citizenship, but you get the point on seriousness of those anniversary dates.