This is from the help file for the Meta Data Source:
To explicitly set the timestamp of a value, set the TIMESTAMP context variable before your return statement. The value to which to set this variable must be milliseconds since the epoch (not a native date). For example:
TIMESTAMP = new Date().getTime();
return p.value + 1;
I haven't tied this but I think you could do something like this to subtract an hour of the current time
TIMESTAMP = (new Date().getTime()) - 3600000;
Keep in mind that Mango doesn't like future dating things to you could have problems if you need to advance the time zone.
I still think this is not the most ideal solution but should work.