Expiration after EAD renewal

Sorry if this is a FAQ, but I have spent hours looking on the web and in the
archives of this group. I found the question posted once but not the answer.

When a renewal EAD is issued from a local immigration office, does it expire
one year after the date of the appointment at that office, or one year after
the expiration of the previous EAD? (or at some other date?)

I ask because all the advice says to apply for renewal from three to six months 
in advance. If the renewal is approved earlier than the old expiration date, 
does that mean the new EAD will expire earlier? In short, is there any 
advantage to applying closer to the expiration date.

This would be EAD under c(9), I-485 pending.

Thanks for any help.