Excel VBA - Function optional parameter DATE type Fails -

i have function finds following monday date. when no optional date parameter passed goes default of zero. think i've included comments explain problem

function nextmondayfromadateortoday(optional startdate date) date ' objective: if date param not supplied should set startdate today ' ??? problem: no arg provided startdate 0 = date value of 1/1/1900 ' ??? how add default optional paramater of date type? ' ??? or, how check if arg provided if parameter date type? if not (isdate(startdate)) startdate = date select case weekday(startdate) case 1:     nextmondayfromadateortoday = startdate + 1 case 2:     nextmondayfromadateortoday = startdate + 0 case 3:     nextmondayfromadateortoday = startdate + 6 case 4:     nextmondayfromadateortoday = startdate + 5 case 5:     nextmondayfromadateortoday = startdate + 4 case 6:     nextmondayfromadateortoday = startdate + 3 case 7:     nextmondayfromadateortoday = startdate + 2 end select end function 

vba variables have default value, , optional parameters automatically set default, behavior expected. since know optional parameter default 0 (1/1/1900) if not passed, why not test value instead of testing see if date passed in?

if, on other hand, think might need pass in 1/1/1900, should set optional parameter variant type. variants not initialized when passed in optionally, not have default value (unlike dates).