I want a button that when clicked will auto-populate a custom date/time field.

[Becky Kaiser]

I need directions on how to create and tie a custom button at the top of a JIRA form to the actual date/time field lower on the form.  This is so developers can click the "Start" button when they begin their activity and hit "Complete" button when they are done, with the button activity auto-populating the Start Date/Time and Complate Date/Time fields.

[Alexey Matveev]

Hello,

You would need to create a transition (from one status to the same status) in the workflow with the Start name and in this transition set the start date field.

You can find how to set a date field to the current date here:

https://community.atlassian.com/t5/Jira-questions/How-do-I-set-a-custom-field-with-the-current-date-time-in-a-post/qaq-p/143415

You should do the same for the Complete Date field