Anyone know how the monthly pass for AT works ? If I buy a pass ahead of my current one expiring, what happens - I effectively "lose" the current pass and start from the new one ? Or is the system clever enough to work out Im just buying ahead ?

ie:
I buy a pass on the 20th of June - its due to expire 19th July. So on the 15th July I buy another monthly pass.  Does the new one now have an expiry of 14th August ? Or the 18th August ?

TIA :)