Standardization

Well we agree on the last sentence anyway, but that also illustrates why both D/M/Y and M/D/Y are bad choices.... Because in the cases where you need the year, it means it figures in somehow and you're putting it last.

If the day is the most important, you just say 'the 15th' or 'Wednesday' or whatever. If the month is important, you can say 'May 15th' or the 15th of May (or, I guess 15 May?). But in the US we literally write like that. If the date is all you need, you say the date, if the month is important you give that info first (since if there is going to be confusion over the month, that's more important) so we say the month, then the date. IF we'd gone further and continued that path and actually wrote it YMD, we would have definitely won the high ground and settled our way as better, but because we decided the year could go at the end, we muddied things up and arguable came in second place...but we came in second place to an almost equally dumb, but one step more consistent, format.

DMY:HMS is still really dumb. Just that MDY:HMS is one step dumber.