问“PT”前缀的持续时间是什么？

EN

P is the duration designator (for period) placed at the start of the duration representation.
Y is the year designator that follows the value for the number of years.
M is the month designator that follows the value for the number of months.
W is the week designator that follows the value for the number of weeks.
D is the day designator that follows the value for the number of days.
T is the time designator that precedes the time components of the representation.

Actually if go on Duration API developed in Java @since 1.8 , they have gone with standard ISO 8601

with java doc as below  :

/**
 * Applies an ISO 8601 Duration to a {@link ZonedDateTime}.
 *
 * <p>Since the JDK defined different types for the different parts of a Duration
 * specification, this utility method is needed when a full Duration is to be applied to a
 * {@link ZonedDateTime}. See {@link Period} and {@link Duration}.
 *
 * <p>All date-based parts of a Duration specification (Year, Month, Day or Week) are parsed
 * using {@link Period#parse(CharSequence)} and added to the time. The remaining parts (Hour,
 * Minute, Second) are parsed using {@link Duration#parse(CharSequence)} and added to the time.
 *
 * @param time   A zoned date time to apply the offset to
 * @param offset The offset in ISO 8601 Duration format
 * @return A zoned date time with the offset applied
 */
public static ZonedDateTime addOffset(ZonedDateTime time, String offset) { }

Obtains a Duration from a text string of pattern: PnDTnHnMn.nS, where
nD = number of days,
nH = number of hours,
nM = number of minutes,
n.nS = number of seconds, the decimal point may be either a dot or a comma.
T = must be used before the part consisting of nH, nM, n.nS


Example of implementation with java as 

import java.time.Duration;

public class ParseExample {

    public static void main(String... args) {
        parse("PT20S");//T must be at the beginning to time part
        parse("P2D");//2 day
        parse("-P2D");//minus 2 days
        parse("P-2DT-20S");//S for seconds
        parse("PT20H");//H for hours
        parse("PT220H");
        parse("PT20M");//M for minutes
        parse("PT20.3S");//second can be in fraction like 20.3
        parse("P4DT12H20M20.3S");
        parse("P-4DT-12H-20M-20.3S");
        parse("-P4DT12H20M20.3S");
    }

    private static void parse(String pattern) {
        Duration d = Duration.parse(pattern);
        System.out.println("Pattern: %s => %s%n", pattern, d);
    }
}