According to the specification in section "10.3.2.3.5 date":
"Where necessary, including the valuedt function (see 10.3.2.x.6), a date value is considered to be equivalent to a date time value in which the time of day is UTC midnight (00:00:00)."

Currently the case time(date("2017-08-10")) returns null, because the evaluation ends with DateTimeException, because internally we represent dates as LocalDate class, which doesn't have time part.