What is your favorite date and time format in a file name? [closed]

I use this:

My-File--2009-12-31--23-59-59.txt

• No spaces
• Double dashes to separate the pieces, making each piece easy to see
• Only one punctuation character (dash) making it easy to type
• No timezone because for me I’m always working in my local timezone; if I needed one I’d go with UTC and append “--UTC” after the time.