Super simple. I would just do a calculated field in a custom form. You'll need to make sure that custom form is applied to the projects you want to see this on.
Here is the calculation:
Here is a doc on creating calculated fields: https://one.workfront.com/s/document-item?bundleId=the-new-workfront-experience&topicId=Content%2FRe...
You could also do a calculated column in a report, but the benefits of having this in a custom form are pretty significant.
You can change the entry date to the actual start date easily. Just replace 'Entry Date' with 'Actual Start Date'
We use entry date because we don't always have a good grasp on when ur projects are actually started, but they're in our court as son as they're entered.