At the bottom of a template's Template Info tab, you will find a table containing records of template creation and modification. The Audit Log table records specific actions and the date and time (in the UTC timezone) those actions were taken. The user responsible for making the changes is also recorded. If no user is listed, it means that the changes were made using your company's primary administrator account.
Click on a column header to sort the log by that column. Click it again to reverse the sort order. Use the Page Size menu at the bottom of the list to display more or fewer records at a time (10, 20, or 50).