it seems to be the java SimpleDateFormat : http://docs.oracle.com/javase/7/docs/api/java/text/SimpleDateFormat.html
here are some tests i did around 11:30pm on the 20th of May 2015
${__time(dd-mmm-yyyy HHmmss)} 20-032-2015 233224
${__time(d-MMM-yyyy hhmmss)} 20-May-2015 113224
${__time(dd-m-yyyy hhmmss)} 20-32-2015 113224
${__time(D-M-yyyy hhmmss)} 140-5-2015 113224
${__time(DD-MM-yyyy)} 140-05-2015