number days-from-date(string)
This function returns a whole number of days, according to the following rules:
xsd:date or xsd:dateTime, the return value is equal to the number of days difference between the specified date or dateTime (normalized to UTC) and 1970-01-01.Examples:
days-from-date("2002-01-01")
returns 11688
days-from-date("1969-12-31")
returns -1