ISO/JSON date format timezone questions

Lars T Hansen lth at
Mon Jul 16 17:13:20 PDT 2007

On 7/13/07, doekman at <doekman at> wrote:
> Hi,
> I was wondering about two things involving timezones.
> The proposal "Date and time improvements" is defining the
> Date.toISO():String method.
> It's unclear to me in what timezone the date should
> be serialized in: local timezone or UTC. Although it's semantically
> correct to allow both and let the implementors decide, but I think
> that would be "not a good thing".
> I prefer is to be serialized in UTC (with the Z suffix).

I agree.  I'll update the proposal.

> The
> downside of this is, all json users will have to support time-zones.

As opposed to the current situation, when they have no idea what the
time stamp means? ;-)

> The advantage is, you can truncate two arbitrary json-date strings,
> and compare them, even if the dates are serialized in different
> timezones.
> The other question is concerning the "JSON encoding and decoding"
> proposal. Is the Date serialization using the (fixed) formatting
> of, or is it using the toISO method,
> mentioned above?

JSON has no syntax for time or date data, according to the grammar on  The code you reference does format dates as strings,
however, and it contains a comment which states that eventually dates
will be formatted according to Date.prototype.toISOString().  This
seems natural to me, as the ISO date format is unambiguous and

So far the committee has been happy not to write any code for
toJSONString, instead (informally) incorporating the code on
by reference.  That may or may not change in the future.


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