>>The function seems short and good
>> However it does not work
>> It returns "invalid argument"
>>
>> I am using VFP 6
>> Is it only for VFP 7 or higher ?
>
>For VFP6, you may have to use the FOXTOOLS.FLL.
>
>SET LIBRARY TO x:\path2foxtools\foxtools.fll
>
>and replace the GETWORDNUM() function with the WORDNUM() function.

...and make sure the dDate is a date or datetime. My usual workaround for resolving version differences in case of these two functions (the other being GetWordCount() and Words()) is to have this somewhere in the beginning

#define v7 "07."$version()
#if v7
	#define words getwordcount
	#define wordnum getwordnum
#else
	do ckfoxtools
#endif

and this somewhere in the end

#if not v7
procedure ckfoxtools
*+ Foxtools Forever and Dark Beer Club. 12.3.2001

if atc("foxtools", set("library"))=0
	cFoxTools = fullpath('foxtools.fll')
	if not file(cFoxTools)
		cFoxTools=forcepath("foxtools.fll",sys(2004))
	endif
	if file(cFoxTools)
		set library to (cFoxTools) additive
	endif
endif
#endif

This works for stuff written in VFP6 or older, so I can keep using words() and wordnum(). For stuff written in VFP7 and later, the #define would have to be reversed; I'd have to
#define words getwordcount
#define wordnum getwordnum
in the "6 or older" branch of the #if...#endif.