2.2.1 [W3C-XSD] Section 220.127.116.11, Order relation on duration
The specification states:
In general, the ·order-relation· on duration is a partial order since there is no determinate relationship between certain durations such as one month (P1M) and 30 days (P30D). The ·order-relation· of two duration values x and y is x < y iff s+x < s+y for each qualified dateTime s in the list below. These values for s cause the greatest deviations in the addition of dateTimes and durations. Addition of durations to time instants is defined in Adding durations to dateTimes (§E).
Some comparisons that are indeterminate are considered to be
true, such as the comparison between 1 month and 30 days.