Change tasks runtime using schtasks

I am attemtping to use schtasks to change a taks run time using the command
line tool "schtasks"

As a test I am attempting to change the time of the scheduled defrag, using
the syntax:

SCHTASKS /Change /TN ScheduledDefrag /ST 15:00

This returns the following error:

ERROR: The specified task name "ScheduledDefrag" does not exist in the
system.

However if I run "schtasks /query " to quey all taks the folllowing is
retured

Folder: \Microsoft\Windows\Defrag
TaskName Next Run Time Status
========================================== ====================
===============
ManualDefrag N/A Ready
ScheduledDefrag 09/01/2008 01:00:00 Ready


Even in the Vista Scheduled taks GUI it shows the tasks name as
"ScheduledDefrag"


What is going on?

.

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