Add the time that the change happened and the value of "a" to my experiment data

OS (e.g. Win10): macOS 10.14.6
PsychoPy version (e.g. 1.84.x): 2021.2.3
Standard Standalone? (y/n) If not then what?:y
What are you trying to achieve?:
hello everyone, I’ m trying to add some time to my experiment data. I define a variable as a, for example. "a"will change all the time during the routine, and when the “a” changed, i want to add the time that the change happened and the value of “a” to my experiment data.
What did you try to make it work?:
屏幕快照 2021-11-15 16.50.09


I want to add some code in builder, but I don’t know where to put it and what the code is be like