Good thinking, but that actually exacerbates the problem I brought up earlier (that some computers aren't internet connected). The block production time is actually a tad unpredictable, since sometimes (in fact, for the last few months) the network hash rate grows so fast that the difficulty adjustment algorithm can't compensate enough to keep the average block production time very close to 10 minutes. Therefore, it's effectively impossible for an offline wallet generator (which is probably one of the most common uses of these wallet encryption algorithms) to get an accurate, or even semi-accurate, block number. But date can trivially be mapped to block height on a networked machine, once the coins are being spent.