The date format for e.g. SINCE is, according to rfc3501:
date = date-text / DQUOTE date-text DQUOTE
date-day = 1*2DIGIT
; Day of month
date-day-fixed = (SP DIGIT) / 2DIGIT
; Fixed-format version of date-day
date-month = "Jan" / "Feb" / "Mar" / "Apr" / "May" / "Jun" /
"Jul" / "Aug" / "Sep" / "Oct" / "Nov" / "Dec"
date-text = date-day "-" date-month "-" date-year
So a valid date is e.g. "22-Jul-2012" with or without the double quotes.