Can I auto-update the Start Date of a Meeting based on a Datetime field in a separate Module?

I am building a Workflow that creates a Meeting under the Account and/or Lead whenever the Demo Date (Datetime) field is populated. Currently, I can have the meeting created, however the Start Date is set to the current Date and Time that the field was saved, and the only option I seem to have is to Set Start Date as X hours from Triggered - no option to populate the entered Date from the Demo Date field in the Leads Module. 

Any assistance would be appreciated.