However: I just tested this by creating a library field with a Date data type: if I type in 2015-12-24 it is converted to 12/24/2015.
I'm no expert on date fields, but I think that's just how they display by default. If you want to display them differently, you can add an expression column and use FormatDate() in that column to show it exactly how you want. If you're going to use that formatting all the time for display purposes, you can define a *second* field like "Validated Display". That second field can use Calculated Data as it's definition. You'd paste your formatdate() expression into Calculated Data, and it would always show you the format you want for the Validated field.
Take a look at the definitions of: Date (month), Date (day), Date (year), and Date (filename friendly). They all use calculated data against the Date field, in order to show us all of these other friendly formatted views that all actually derive their values from the Date field.
Brian.