if you are booking for 17th may, you only can book on 16th Feb.
Rule says 'you can book 90 days advance, excluding the day of journey'
for your case 17th may you want to travel, you count backward as under
16 days of may
30...
more... days of april
31 days of march
13 days of feb :
---------
90 days
-----------
So on 16th Feb the ARP : advance reservation period for booking your ticket opens.