Does UTC date string require the format specifier "Z" even if that string includes time offset?

Question

For instance,

const d = new Date("2012-08-20T15:00:00-07:00");

d here is a UTC time with time offset = 07:00. Does it still require Z like this 2012-08-20T15:00:00-07:00Z? Is this correct?

If I take this string with Z and parse it using Date.parse() method in JavaScript, it throws an error. Not sure what is wrong!

Answer 1

No, you should not include the "Z" with a time zone offset.

From rfc3339:

  Z           A suffix which, when applied to a time, denotes a UTC
              offset of 00:00; often spoken "Zulu" from the ICAO
              phonetic alphabet representation of the letter "Z".

The "Z" is a zero time offset, so including it with an explicit offset (especially a non-zero one) doesn't makes sense.

Answer 2

Quoting W3C note on Date and Time Formats:

YYYY-MM-DDThh:mm:ss.sTZD (eg 1997-07-16T19:20:30.45+01:00)

where:

[...]

TZD  = time zone designator (Z or +hh:mm or -hh:mm)

Notice the or word above. You either specify time zone offset or Z for Zulu (no offset).