It is apparently 5 years since Daphne moved in with the Cranes and started working for them. She moves in in September 1993. It is May 1998 in this episode--a few months short of 5 years.
It is of additional interest to note that Roz discovered she was pregnant last Halloween and if this is supposed to be Daphne's anniversary (September), Roz should have given birth by now. It seems that broadcast dates can't be applied to "Frasier time" too rigorously.