Version: (using KDE Devel) Currently, the only way to get the date from within a XSLT chat style is by using @timestamp. There are various problems with this, such as the lack of seconds on @timestamp and the inability to style date separately from time in a style.
Kopete 0.12 will move away from XSLT to using the Adium format for chat window styles. The adium chat message style definitions seem to have everything you need to do things with timestamps, dates, etc.