seeking extention for joining date from PSU

sir, i got appointment letter from PSU stating the training date/place of posting/joining date after clearing all india exam, interview n medical.. but, i need extention of 45 days as iam retiring from armed force after completing 15 years. i had mentioned my date of discharge at the time of interview and again at the time of medical after final selection. my NOC from armed forces clearly mentions my date of retirement, but suddenly the PSU has released the training schedule and within 02 weeks ask to join us, else states our candidature will be cancelled. the PSU has not mentioned any time frame earlier at the time of advertisement. now at this short notice time how we can join , so we asked extension but company is not in favour of this , so pls guide us what can we do i had filled the required notice period of 03 month for joining if selected when asked by the company at the time of interview, medical n every formality mentioned. so at this short notice period of 02 weeks is not possible, as every company has to publish the final date at the time of advertisement or should give atleast 02 months of joining time so that we can get relieving letter from our prvevious employee. n in my case its very clear i mentioned my date of discharge from armed forces n then only i can join the PSU .