how to edit of WI updates condition

even if you get it to record the time only on a state change (this is currently not possible as a save will invoke the script forcing it to return some value and hence inducing a changed attribute value), if the worflow allows a reopen (and possible revisit of the same state), the condition will be true again and a possible second assignment of the attribute value...

Hi Ayan,

Any save operation will trigger the execution of scripts. 

Below is an extract from Attribute Customization Wiki which states the three conditions where a script gets executed:

If you configure a Script-based calculated value, the script will be executed in three cases:

• When a work item is created, the script is executed
• When a work item is saved, the script is executed to recalculate the value.
• When an attribute which the current attribute depends on is changed, the value of the current attribute is recalculated. Depending on the presentations of the two attributes this may not work in all cases.

hence the behavior you are experiencing appears to me to be expected.

Then this is not doable with JavaScript. JavaScript has no mechanism to detect the state change as far as I can tell.

You need to create an Advisor or Participant.
See https://jazz.net/library/article/1000 or the other versions for your version of RTC and check https://rsjazz.wordpress.com/ for some other examples.