The "correct" direction for mobile safari would actually be left, IMHO. Mac and iOS (as well as the iPod and watch) use navigator patterns heavily, where left to right you have a stack of screens with progressively more detail. So it would make sense to have a MMYYYY view, which opens into a monthly calendar, which goes to a time of day.

"Correct" in quotes though because the date/time information is directly manipulatable at every level (it is a picker, after all). Turns out the best level to start at and affordances to pick a date in particular differ by use case.

For instance, the iOS Calendar app has a years view, but it shows a full yearly calendar, one per screen, and makes you swipe 15 pages up to see if you had any calendar entries from the first iPhone.

This makes sense from a calendar point of view, because it is far more common to want to navigate within the same year or previous year than to want to visit your calendar from 15 years ago.

It would be an absolutely terrible interface for seniors to enter their birthdate; far worse than the default here in fact.

If you enter a birthdate in the Contacts app, you get the "three tumbler" interface (for Gregorian calendars, at least) - months, days, and years, presumably in a localized order with localized text.

It is somewhat unfortunate they didn't just stick with this for the iOS Safari interface, as form input entry already expects that a good chunk of screen real estate will disappear for the keyboard. Instead, they went with a floating picker because thats what most websites currently implement (via javascript)