AppleScript
@bocciaman, from your description (always better to share the actual action by the way as the picture is incomplete without it) it sounds like you are inserting your template and then just running some AppleScript after it expecting it to operate on and update the inserted text. I am not sure if that was your intention, but that is not how it is designed to work and would explain why it did not work for you.
Instead I think you would want to run the AppleScript, take the output of the AppleScript, and insert it into the editor at the current cursor position.
Here’s a version that does that.
PLEASE NOTE!
I have modified this particular version to work backwards by 5 and 6 days for Saturday and Sunday respectively to allow for testing this over the weekend. Simply remove those four lines (condition and action x 2) to restore the original script structure.
JavaScript
It is pretty easy to do some date calculations in JavaScript in Drafts. Greg posted a straight forward example earlier this year in response to a question about it.
TextExpander
I have an idea for TextExpander that could workaround something that came up earlier, but there’s an issue with it when I came to put it together. I’ve had to contact and open a support ticket with Smile as I think they have broken/removed something that used to work and would be helpful for a more generic solution.