Problem with date comparison for custom fields

This:

date( 'Y-m-d', strtotime('-6 hours') );

Is going to spit out something like 2011-11-11

If your date picker is using the format dd-mm-yy, then comparing the two is not going to work. dd-mm-yy, if it’s what’s in the field on post save/update, is what’s going to be saved as the meta_value.

The meta value is likely getting saved as a string, which is fine — you can still do date comparisons, but the formats have to match. There are a few options for solving this:

1. Convert your dates to unix time stamps before saving them in the
   database — use strtotime — do the reverse on
   the front end: date( 'F, j Y', $some_time_saved_in_db ); to show the dates to users. Comparing timestamps should be the most reliable.
2. Save your field with the ISO standard date format, YYYYMMDD, which
   works fairly well for comparing/sorting dates.