%{issue.sprintStartDate}
Read
If the issue is assigned to an active sprint, the value of this sprint will be returned.
If the issue is related to several sprints, the value from the currently active sprint will be returned.
If the issue is related to several sprints and all sprints are inactive, the value from the sprint completed most recently will be returned.
When cast to text, the output depends on the jira.date.time.picker.java.format
parameter set in your Jira instance.
19/Mar/20 1:38 PM would be returned for dd/MMM/yy h:mm a
If you want to return a number (a timestamp) instead, remove the leading % character in front of the field code.
You will not be able to do this in the basic mode. Learn more about Parsing modes.