That is correct, we followed what they had documented, the String parameters must have been an undocumented feature. From an app however you can use the toDateTime(String) method to parse your strings into Dates, ie:
def between = timeOfDayIsBetween(toDateTime(fromTime), toDateTime(toTime), new Date(), location.timeZone)
toDateTime(String) takes a String in the format: yyyy-MM-dd'T'HH:mm:ss.SSSZ
