MarketClock — Trading Days API

Built in October 2026 across 24 pull requests: a pure-Python date layer on the holidays library with 100% branch coverage, a Flask API with Pydantic validation, MySQL for sourced settlement cycles and official calendars, and Redis for caching and rate limiting. The Next.js web app has a page per market and year, a yearly index of every exchange, settlement, overlap and business-day calculators, calendar subscriptions, an embeddable status card and a data-quality page. Every fact that doesn't come from the library, a session hour, a settlement cycle, an official holiday, carries the link to the page it was read from. 172 backend and 38 frontend tests run on every pull request.

1. 01

   Sourced, not guessed: settlement cycles, trading hours and official calendars are stored with the exchange or regulator page they came from, and a market with no confirmed source says so instead of inventing an answer.

2. 02

   Checked against the exchange: NYSE, Nasdaq and London Stock Exchange calendars were compared day by day with the library, 66 official days in all, and every one matched.

3. 03

   Open right now: each market's state is worked out in its own time zone, with a countdown to the close or the next open, including early closes before holidays.

4. 04

   Settlement that respects history: a trade dated 2020 settles on the old T+2 rule and a trade dated today on T+1, because cycles are stored with the date they took effect.