I added a 2 byte 'weeks since 2013-01-01' field and updated the prefixes, ranges and test vectors. The updated proposal lives here: https://bitcointalk.org/index.php?topic=258678 Cheers, jp On Jul 22, 2013, at 06:14 AM, Mike Hearn wrote: This isn't usable for SPV wallets unless it has a birthday in it. Otherwise you either need to scan the entire chain (slow) or find a fully indexed copy of the block chain (expensive, more centralised). Just add a UNIX time as an extra 4 bytes, or if you want to save a few characters then use a uint16 that represents "days since birth of this specification".