Think it is in the FAQ.
Q2. Can I book for IPT anytime within my IPPT window?
A2. NO. You will have to book for an IPT schedule that ends within 9 months after your birthday. This would enable NSmen who could not pass their IPPT at the end of IPT to continue their own training, applying the training methodology to improve on their performance and thus maximizing the knowledge gained during the IPT.