Finding a timestamp of a work Item State change.
One answer
There are many similar discussions in jazz.net which you can refer to:
https://jazz.net/forum/questions/94742/assigning-current-time-stamp-upon-closure-of-defects
https://jazz.net/forum/questions/101585/how-to-set-the-value-of-custom-attribute-of-type-timestamp-to-nullnone-using-javascript
https://jazz.net/forum/questions/162999/assigning-current-time-stamp-when-defect-is-resolved
Hopefully this helps
https://jazz.net/forum/questions/94742/assigning-current-time-stamp-upon-closure-of-defects
https://jazz.net/forum/questions/101585/how-to-set-the-value-of-custom-attribute-of-type-timestamp-to-nullnone-using-javascript
https://jazz.net/forum/questions/162999/assigning-current-time-stamp-when-defect-is-resolved
Hopefully this helps