You have posted that:
"I have been issued relieving letter of a private firm (where I worked) of date 15 days later than I joined the Govt Service. As I didn't had the relieving letter at the time of joining, I was asked not to show the experience at that time. In the meantime (after 1 year), I got the fresh appointment in other govt dept. So, here, Should I show the experience of private firm (given overlap of some days b/w private org and 1st govt service)?'
1. Probably you mean that you joined govt service say on 1st of the month and the relieving letter issued by private company shows that you were relieved on 15th of the month.
Is it so?
If yes does the relieving letter states clearly that you completed some formalities on 15 th of the month hence this date is mentioned otherwise your last day/date in office-employment was say anyday before 1st of the month?
If NO; you are showing 15 days parallel employment with a private company while you were with a govt. establishment.
You can be subjected to disciplinary action.
2. Who asked you not to show the experience at that time? Were you asked in writing?
Even if you are asked in writing you have concealed and you can be subjected to disciplinary action.
3. You can not and must not conceal anything.
You should have got the error corrected in Relieving letter immediately.
Rest is upto you.