Setup default date format like yyyy-mm-dd in Powershell?

A simple & short question:

How can I setup a default date format in powershell like yyyy-mm-dd ? so any date output will be like this format?

or How to setup a date format globally in one script ?

Is there a way to output date only without time? when I output LastWriteTime, Default is

13-03-2014 14:51

I only need 13-03-2014, without the 14:51 part.