For anyone who's been having trouble working with the "accountexpires" attribute in Active Directory after having read the following article 

www.microsoft.com/technet/scriptcenter/
resources/qanda/sept05/hey0902.mspx 

or something similar, this may save you some frustration. In the article is is mentioned that this attribute is an integer representing the number of nanoseconds since 01-Jan-1601 00:00:00.

However the "accountexpires" attribute actually seems to be the number of 100 nanosecond increments since 31-Dec-1600 14:00:00. As a result if you divide the integer by 10,000,000 and subtract 11644560000 you will get a Unix timestamp that will match the dates in AD. 

To set the "accountexpires" date just reverse the procedure, that is, get the timestamp for the new date you want, add 11644560000 and multiply by 10,000,000. You will also need to format the resultant number to make sure it is not outputted in scientific notation for AD to be happy with it. 

Hope this helps!