Formula looks fine to me. Works in my test app. Pardon the question - you've reproduced it here with an errant trailing comma - is that character in your formula?
EDIT to clarify: this formula works if [Date Invoice Sent To Client] is a date/time or text field. It will NOT work if that field is a simple date field. However, if that were the issue you'd be seeing this error:
"Formula error -- Bad or missing arguments in function call"
rather than
"Formula syntax error -- There are extra characters beyond the end of the formula"
Thanks to @Xavier Fan for narrowing that down