You're welcome.
The dates passed to the BEFORE, ON, SENTBEFORE, SENTON, SENTSINCE and of course the SINCE query are always in the formatdate-day "-" date-month "-" date-year as described in the formal syntax part of the specification.
It's really not a pleasent thing about IMAP, a pity you can only filter letters by date and not by time. Can be critical if you deal with big amounts of data.
Coming to the FormatException you mentioned - I need more details about it. You can post a stacktrace, or try sending me an exported *.eml file top13a92@googlemail.com. The parsing alghorithms are still not ideal and we work on improving that, so all kind of feedback and bug reports are welcome.