So let me get this straight (since it's more immediate for
you).
You have 20 days before your window closes, but you also have a
valid medical excuse yes?
Technically, you should be safe. Technically you won't be
charged as an IPPT defaulter.
However, because this is NS, murphy's law applies. Anything
wrong that can happen will happen. Do not be surprised if you
received a letter charging you for defaulting even though you have
a medical excuse.
If this happens, remember to appeal. This is a grey area which i
think an MO can clarify better than me. Since you are on excuse,
you are not medically fit to take ippt. However, i think mindef's
guideline is that a person can only be excused ippt for the year if
he has 6 months cumulative MC or 9 months of non-cumulative mc.
The situation might be that mindef can say why for the 11 months
when ur window opened, you did not attempt your ippt, why you wait
till last minute. however, u can also argue and say that you have
many commitments that prevented you from taking ippt.
you have 20 days or less before your window close, i suggest you
call ns.sg, call your unit, do whatever it takes IN YOUR power to
get an answer. i.e. unit clear you for ippt for the year.
anyway, just refer to the thread that eac linked. you can read
more about my/other forumers' experiences.