Unix timestamp: seconds since 1970-01-01 00:00:00 UTC. Does not include leap seconds. 32-bit signed overflows on 2038-01-19 03:14:07 UTC — the year 2038 problem. Use 64-bit (or millisecond precision, 64-bit) to be safe.
ISO 8601: 2026-04-18T14:30:00.000Z — the unambiguous string format. The T separates date from time, Z means UTC (Zulu), or +05:30 means offset from UTC.
IST is UTC+5:30. There's no daylight savings in India (single time zone despite the geographic width). The half-hour offset comes from compromising between Calcutta (UTC+5:53) and Bombay (UTC+4:51) time.