Time-limited challenges run inside a fixed window, usually a calendar month or an anime season. Every title has to be started and finished inside that window, and the deadline is in UTC, not your local time.
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Most monthly challenges offer an Easy mode (a mandatory requirement plus a few of your choice) and a Hard mode (every requirement, often with a total runtime minimum). Seasonal challenges ask for anime that started and finished airing in that season, with a submission window a few weeks after it ends.
Tip. Pick titles that cover several requirements at once. A series with 20 or more episodes can often fill two slots, which matters when the clock is running.