When text object name is timestamp, we support a copy of text object default for this name. Basically, it will need to contain all text, position, frame related format except text content.
When user click Date&Time Stamp button, we create text object with this default.
No matter which layer user selected, we only create Date&Time stamp for 1st layer. Since we will update when there is existing time stamp, so active other layer will just update time stamp of 1st layer.