Date calculators look simple until the rules around the date become the main product. I have been using a small court-day calculator as a reminder to keep the implementation boring, explicit, and easy to audit.
The most important design decision is to separate ordinary calendar math from jurisdiction-specific legal rules. A tool can help someone count days, compare deadlines, and avoid manual arithmetic mistakes, but it should not pretend to replace local court rules or legal advice. That boundary needs to be visible in both the interface and the page copy.
For the calculator workflow, I keep a few implementation notes close to the code:
Normalize all input dates before doing any counting.
Show the start date and end date in the result, not only the final number.
