samir vi,
Yes, just give a date calculate from date of issue and mention in the
cover letter that " National ID of bangladesh is valid for 15 years"
Nice to know:
Expiry date of national ID card is not a big issue. Many countries dont have national ID card; for instance: our loving country CANADA doesnt

Moreover, many countries(ex: Greece) National ID card dont have expiry date likewise Bangladesh.
Source: http://en.wikipedia.org/wiki/Identity_document#Canada
BR
Dr.Hasib
I talked with one of my relatives in Election Commission and he said, everyday many embassies do query regarding NID matter and Election Commission reply accordingly via letter, email etc. But about expiry date, it is surely 15 years from date of issue and there is no risk to put it in our Generic form. Hope they will not verify this silly issue and if they, definitely the commission will help them. NID is basically verified for those who are international criminals.