Jean-Paul, it may be worth noting that the BIP39 word list is integrated into Bitcoinj so will likely become the de facto standard for Android, Trezor web and several desktop wallets. Anyone deviating from that word list would likely find themselves in an isolated pocket.

Regarding the timestamp, MultiBit HD uses a simple timestamp of "number of days since midnight of Bitcoin genesis block in UTC with modulo 97 checksum appended". Thus a new seed generated on 27 January 2014 would have "1850/01" as its checksum.

I'm a bit confused, are you changing the way the checksum is calculated, or fudging the input seed to produce a specific checksum? Or is checksum in this case another value calculated over the mnemonic list?

When creating a new wallet the users are tested that they have written the timestamp down along with the associated 12/18/24 words.

So this is specific to MultiBit HD? Wouldn't it be better to include this into the BIP?