Caching header with mongodb timestamp

Found this issue when testing caching using mongodb as our data storage. The last-modified date is being correctly, but it by its specification it doesn't include milliseconds thus when the node JS controller tries to compare them they are different:

Fri Feb 27 2015 09:15:00 GMT+0000 (GMT) - Mongo DB date
Fri Feb 27 2015 09:15:00 GMT+0000 (GMT) - if-modified-since

Controller values:

1425028500987 - Mongo DB date
1425028500000 - if-modified-since

As you can see the mongodb includes milliseconds. Has anybody a nice way to clean up the date to reset milliseconds?

Thanks.

J