Durations define the amount of intervening time in a time interval and are represented by the format Por P[nW as shown on the aside. In these representations, the is replaced by the value for each of the date and time elements that follow the [n. Leading zeros are not required, but the maximum number of digits for each element should be agreed to by the communicating parties. The capital letters "P", "Y", "M", "W", "D", "T", "H", "M", and "S" are designators for each of the date and time elements and are not replaced.