It works. And, as anyone who ever used them to pick a date and time different from the prepopulated one knows, it is painful to use it, even if you don't resort to the click mayhem of doing it keyboard unaidedly. That's six times two clicks plus two or three click+drags to scroll among the options of the larger select boxes plus all the precision mouse manouvering involved with hitting the very small target zones for each and every one of those clicks. Try it with a laptop mouse pad if you find it too easy with a mouse and your many years worth of computing experience. It's just not fun any more. Add the joys of converting 24 hour time to AM/PM if your brain isn't wired to twelve hour time for another optional tinge of discomfort.
What I'm trying to say is that it is not a very friendly approach to achieving the wanted functionality, though it admittedly makes it difficult (but not impossible!) to enter illegal date/time combinations. So, how would one improve upon the situation without sacrificing that criterion? After all, replacing these select boxes with a pre-populated text input reading a default date, say on the format "YYYY-MM-DD hh:mm" (annotated as such), might do away with the click mayhem involved in picking a date and time combination of your own, but it instead becomes easy to make a mistake that would require another iteration of editing, in case that date somehow turned out not quite up to specs. Especially as that format is quite likely not how the user would intuitively phrase a timestamp in writing. Another server roundtrip (for a non-ajax typical implementation) for an "uhm, try again with the date, please?" also does not equal usage bliss.
But the text entry is a great place to start, and for some use cases much better than would a popup calendar date picker be, for instance.
Aside from the specifics of this (short of perfect) visual packaging, try on this Blogger free-form date field user script (direct install link) and head over to your Blogger post editor, where you will now encounter a variant, which leaves the fields to show which date has been picked and lets you type dates by hand in a text field next to the dropdowns, updating the rest of the fields with the data it successfully understood.
While mostly crafted for my needs of half a year ago, when I was importing a lot of blog posts from an external source that listed dates formatted in a textual style I'd want to just paste and be done with, I took some time to make it a bit more useful, understanding lots of partial and complete date formats, in English, French and Swedish (I don't do too well with other languages). It is rather easy to extend it to others, as long as there is no overlap in the names or short forms of months and the relative dates "today" and "yesterday" among the languages listed.
Note how it clues you in to which parts of the date "took" by tinting the appropriate fields greenish. As the Blogger date fields only suggests a few recent years in the year dropdown, I spiced it up to liberally add whichever year you type in yourself, too, after a slight delay, in case it was a typo just as quickly fixed as it was typed. I don't recall the exact details around why I found that safeguard useful any more, but left it in place, in case there was some good thinking behind it. There often is, but I didn't seem to clue in a good enough documentation trail. Anyway, a sample of date and time formats it can handle to play with, to get you started:
12/25/2005 (read commentary below if you want D/M/Y dates!)
6 Aug, 1996, 04:28
samedi 28 février 2004 19:55 (irrelevant junk around ignored)