I typed the following all out because at first I didn't immediately agree as that was not the intent of the offset.
However, I can see it's easier to understand that way now ^^

Right now there are only 2 more values to worry about `duration` and `offset`  (ie: no more units, as everything is specified in seconds)

#### duration
duration is the maximum amount of time that the signature should be valid for.
ie: If you specify a duration of 10 days, the signature you get, **can** be valid for 10 days, but it can just as well be 6 or even 5 minutes.
It all depends on how many multiples of 10 days you're ahead of epoch (January 1, 1970)

#### offset
Offset is an amount that will always be tacked onto the previously calculated expiration date.
It's something to avoid the problem of handing out a signature that will expire in 3 ms (as that result can occur in the previous step) and trying to use it 200ms later in an AJAX request.
ie: If the expiration date is 25th of January 13:00 and you specify an offset of 1 hour, the signature will expire the 25th of January on 14:00.


Now, with all that said, I think we could have the following settings:
* Profile pictures
    * duration: 1 month
    * offset: 1 day
* Content previews
    * duration: 15 mins
    * offset: 15 mins