$Script Syntax Error

The following script in a text field display the timer value in minutes:
[$SCRIPT=&nhs.TimerValue("MaxTime").TotalMinutes()]

The following syntax does not work to subtract the timer value from the variable "MaxTime".

[$SCRIPT=&(nhs.GetVar("TestVariable")-nhs.TimerValue("MaxTime").TotalMinutes())]

Any help would be much appreciated to get the syntax correct

Thanks,
Wallis