JFS Support & Maintenance renewal policy is ruled by the clause 7 of the JFS EULA. According to it: Subsequent Renewal Periods commence upon the expiration of the prior Software Maintenance regardless of when it is purchased. This means that upon purchasing of, say, 1-year renewal, this 1 year will be added to the current expiration date of your support & maintenance period and not to the current calendar date. See example below.
JFS license model is in general just the same as the license model used by Atlassian for all their products. As for the clause 7 you may find the same statement here: https://www.atlassian.com/licensing/purchase-licensing#softwaremaintenance-3
Example: suppose today is 01/Jan/16 and your existing support & maintenance period expired on 30/Jun/15. Then upon purchasing a 1-year renewal the resulting expiry date will be 30/Jun/16 (30/Jun/15 + 1 year).