I think that can allow for the system to be games. If the total you are seeking is 100 HBD, at 1 HBD/day, with a start date of a month away, and you get enough support on day 1, you can just move the start earlier and get an extra 30 HBD. Now its up the the voters to unvote if they notice irregularities, but whats to say everyone is keeping an active eye on it? I think this is a no.
Moving it later on the other hand, that I do support.
But what if by changing the start date, the end date is also shifted accordingly?