Block number is important, to lock the state of community members who are able to vote. Meaning that if you attempt to vote on a proposal and block number is in the past, and you weren't holding required token yet, your vote will not be counted.
Explained in depth:
H = h + ((t1 — t0) / a)
Where:
H
= target block height
h
= current block height
t0
= current timestamp (in seconds)
t1
= target timestamp (in seconds)
a
= average time to solve a block (in seconds)
Or...
last_block_number + ((future_time - time_now) / block_time)
So, for example, using a current epoch time of 1481214124, the epoch time of 1482537600 for midnight Christmas Eve, and the last block of 2771338:
2771338 + ((1482537600 - 1481214124) / 14) = 2865872