Build a small offline time-zone planner in HTML, CSS and JavaScript.
Take an ISO timestamp with Z or an explicit offset as the source instant.
Show the full date, weekday and time in Europe/London, America/New_York
and Asia/Kolkata using date-aware Intl.DateTimeFormat handling.
Reject invalid or zone-free timestamps with an explanation.
Label the source instant and each named zone. Do not use fixed offsets.
Keep it local, with no calendar connection, tracking or invitation sending.
Return the actual downloadable files and instructions for opening them.

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Check the actual planner with these two instants:
2026-10-20T13:00:00Z and 2026-10-26T14:00:00Z.
Compare the displayed full date and time with the supplied expected values.
Also test a date crossing midnight in one zone and an invalid input.
Explain how the implementation avoids a permanent five-hour subtraction.
Separate observed file checks from any assumptions about future zone rules.

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The source local time can refer to more than one instant. Do not choose
one silently. Ask for the intended offset or an unambiguous timestamp,
then show that source instant beside the zone conversions.
Repair the input validation while preserving the working date-aware display.
