Hi Mark, Yes it is possible to use Parameters in Email Subject fields.
In order to use a Solution Parameter containing code we need to first interpret the code in the flow and can then use the parameter value in the Email subject field.
In your flow, Add a Calculator step, Click on the paper clip icon and select your parameter
The calculator will return the object as per the below
In the EMail destination step, specify the Subject as per the below image
@record.subject
Please note when working with Dates in Synatic, You can make sure of the Day.js library, For more information, please refer to the below links
Using DayJS in Calculators - FAQs / Calculators - Synatic Community
Let us know if you have any further questions.
Thank you
Faizel