Flyspray has date and time fields at various locations, like the search box and on the edit task page.
You can use the calendar icon to open a Javascript based calendar to choose a date or you can type in a date manually. Flyspray will accept a wide variety of formats. The recommended one is YYYY-MM-DD (like 2006-12-24) or YYYY-MM-DD HH:MM:SS (like 2006-12-24 13:50:23). Other possibilities are relative dates like ”-1 week” or ”+4 days”. The full syntax can be read at http://www.gnu.org/software/tar/manual/html_node/tar_109.html.
Also, make sure that the time zone in your profile is set correctly. Flyspray considers this time zone both when inputting and displaying dates.
You can specify a date format both in your profile and the admin area. The syntax is the same as strftime() uses. The default short format is ”%Y-%m-%d”, the extended is ”%A, %d %B %Y, %H:%M %GMT”. Note that %GMT is a Flyspray specific paramter.