/DU duration

A value that specifies the duration to
run the task. The time format is HH:mm
(24-hour time). For example,14:50
specifies 2:50PM. This is not
applicable with /ET and for the
following schedule types: ONSTART,
ONlogoN,ONIDLE,and ONEVENT. For /V1
tasks (Task Scheduler 1.0 tasks),if
/RI is specified,then the duration
default is one hour.

Windows XP: This option is not
available.

Quoted from MSDN.为什么TechNet Library page for schtasks中没有这些信息超出了我的范围.