I would interpret it as pick up by April 30th. Example, on their current promo page, the 2400 pt promo says...
"Offer is valid on all car types for rentals through May 31, 2016"
Then below in the T & C's....
"rent any vehicle by May 31, 2016"
Meaning pick up by. I have found this to always be the case, as opposed to completing rental by that date. YMMV.