I have several Ferry-Morse seed packets, which, on the back, show what the expected "Days to Harvest" is. I am wondering if this "Days to Harvest" should be measured from the date I planted the seeds or the date I saw the sprouts first coming up?
It varies by type of plant and sometimes between seed sellers. It could mean from when you sow your seeds or when you transplant. Burpee is one of the seeds merchants that is pretty clear about what they mean and in general appears to follow the following pattern:
When seeds are primarily directly sown in your garden the maturity date is from the time your seeds are sown. That appears to be a common way of presenting maturation.
For plants that are started indoors and then transplanted out in your garden after a later date maturity is measured from the transplant date.
Here's an example of lettuce that describes date to maturity as "25 days after sowing.
Here's an example of cabbage that describes date to maturity as "60 days after setting plants in garden".
Ferry-Morse isn't clear on what they mean by maturity and there isn't much info on their website. If you can compare the seeds you have to someone that is clear, like Burpee, you can probably figure out what they mean by maturity.
I recommend calling the telephone number on the package, and asking for directions about your specific seeds. Park Seed offers the option to speak with a horticulturist or a master gardener. If you have any questions about something they sell, even if you haven't purchased it from them, they're happy to answer questions and teach techniques to help ensure successful results.
Their time-to-harvest instructions on the package are generally clear and follow the same basic pattern explained by @OrganicLawnDIY. Since there are many variations, even among the same vegetable, a phone call can also be beneficial.