sum    : PT1M + PT2M
result : PT3M
sum    : P1DT2H + PT3M + PT4S
result : P1DT2H3M4S
sum    : PT20M + PT20M + PT20M
result : PT1H
sum    : PT01S + PT02S + PT04S + PT8S + PT16S + PT32S
result : PT1M3S
sum    : P1M
result : P1M
sum    : PT70M
result : PT1H10M
sum    : PT70M + P13M
result : P1Y1MT1H10M
sum    : PT70M + -PT30M
result : PT40M
sum    : PT1H10M + -PT30M + -PT30S
result : PT39M30S
sum    : PT1M + -PT1M
result : P0D