Before:
The picture below is from an DateTimeInputWidget (Firefox 58.0 on Linux, red line added by me for emphasis):
As you can see, numbers are a few pixels above the line on which letters are aligned. Looks untidy, and triggers OCD!
Pages with DateTimeInputWidget:
(Note that other pages (Special:UploadWizard, Special:Contributions) actually use a DateInputWidget, which for silly historical reasons is a completely separate widget: T91148)
After:


