An Italian provider in his webservice documentation defines a date (birthDate) as string. Why ?
I discovered the format provided is d-m-Y, my database store it as Y-m-d and my users prefer d/m/Y (as many Italians).

ISO format is the only real format: YYYY-MM-DD.

You can sort ISO format as strings and it just works!

If the transfer format serializes dates to strings (e.g. JSON) string probably is the only valid type for a date.
The alternative would be integer when storing the unix timestamp.

@sbiewald works if you're born after Jan 1 1970.

Hold on, are you telling me people still uses Date, Time, String or any kind to express a date which is not a fucking LONG number!!!????

Jesus freaking Christ, what are you? animals?

@olback Negative numbers?

@arcioneo so you haven't dealt with 1969 before?

DateTime is generally a string

YYYY-MM-DD HH:MM:SS
OR
YYYY-MM-DDTHH:MM:SSZ

Unless it's Unix time stamp (int)
But then you may end up with the 1969 issue of hitting a negative value.

The format can change but it's usually the same.

ISO-8601 is your friend.