presto-main/src/main/java/io/prestosql/operator/scalar/DateTimeFunctions.java/DateTimeFunctions/diffDate
class DateTimeFunctions:    @Description("difference of the given dates in the given unit")
    @ScalarFunction("date_diff")
    @LiteralParameters("x")
    @SqlType(StandardTypes.BIGINT)
    public static long diffDate(ConnectorSession session, @SqlType("varchar(x)") Slice unit, @SqlType(StandardTypes.DATE) long date1, @SqlType(StandardTypes.DATE) long date2)
    {
        return getDateField(UTC_CHRONOLOGY, unit).getDifferenceAsLong(DAYS.toMillis(date2), DAYS.toMillis(date1));
    }