PE TO SUBSTRACT TO DATES.

Hi all,

I am sure it could be a very easy question but i don't know how to reach the solution.

I would like subtract one date value stored in a PE named TEST_DATE(Point Type is a string as you can see below) with other timestamp values.

Evaluating currently  this  sentece 't-1s' i get 24/10/2014 1:25:40. It's ok, however if i try to do something like this:

 - 'TEST_DATE'-1s 

 -  ParseTime(Concat('TEST_DATE', '-1s')),

 - ParseTime('TEST_DATE') -1s,

 - ParseTime('TEST_DATE'-1s ),

i got call Failed or Performance Equation parsing error

although it's not neccesary you can view TEST_DATE archived  values

Thanks in advanced!