date +%FT%T%Z

その他使えそうなやつ

ISO8601 UTC timestamp           | date --utc +%FT%TZ         | $(date --utc +%FT%TZ)
ISO8601 UTC timestamp + ms      | date --utc +%FT%T.%3NZ     | $(date --utc +%FT%T.%3NZ)
ISO8601 Local TZ timestamp      | date +%FT%T%Z              | $(date +%FT%T%Z)

Most common Bash date commands for timestamping « The Intellectual Wilderness