Steam Change Date Format
Solution 1:
Short answer: No, there is not.
Some kind of explanation: This topic comes up in the steam community from time to time, but the developers really doesn't seem to take any notice of it. I seems like steam treats american English (and it's timeformat) as the standard, and therefore does not want to change the way their date formatting is set up.
Some report that steam should adjust to the way your computers calender is set up. But as far as I can see, it seems like the ones that are saying this, are people from the states (and their computers calender is therefor the same as steams).