Resto Druid Rejuvenation is now modified by the optional level 100 talent Germination.
If you take Germination, your Rejuvenation now stacks twice. The second stack has a different spellID and maintains a separate timer.

http://www.wowdb.com/spells/155675-germination
http://www.wowdb.com/spells/774-rejuvenation

two decent possibilities for use case -

1. only display the countdown of the lowest of the 2 buffs (which is the stack that will get time added)
2. show both timers on the button

2. is impossible. 1) might be doable.