Hi @ChrisC,
You need to take use of the Workflow Definition Language to deal with that.
Could you please share about the previous steps, better with a screenshot of your flow?
Using WDL would need to translate the Dynamic content into content that could be understood by the language.
For example, if the previous step is "When a new event is created" (I suspect),
Then we need to change the Body input of the Send an Email action with the formula below:
"@{Adddays(triggerBody()?['StartDate'],-14,'MM/dd/yyyy')}"
if the previous step is not a trigger, then I would need more information to make it work.
Adding the workflow Definition language as the reference:
Workflow Definition Language
Regards