Date and Timezone Handling

Help me handle dates and timezones correctly for {{use_case}} in {{stack}}. Recommend how to store timestamps (UTC), when and how to convert to the users local time, handling daylight saving, dealing with date-only versus datetime values, user-entered times, recurring events across timezones, and display formatting. Provide the approach and the pitfalls. Date and timezone bugs are notoriously common and subtle, so note the specific mistakes that cause off-by-one-day and wrong-time errors and how to avoid them.