You can find in PIDatalink, or maybe in a trend at PIVision or ProcessBook
The best for me will be saving in another PIPoint timestamp last change in TAG because my user use this information in another system and he can not use trend or PIDatalink
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If I understand the question, you want a tag to hold a timestamp every time another tag value changes.
The first step is creating your new tag with a point type of timestamp to hold the value change time.
Then, if you are using Performance Equations, a tag with an event source in the extended descriptor:
event=<Source Tag>, if '<Source Tag'<> prevval('Source Tag', '*') then '*' else nooutput()
If you are using AF Analytics, create an analysis that is event triggered on your source tag:
if '<Source Tag'<> prevval('Source Tag', '*') then '*' else nooutput()