pre-1.2.0
A value cannot be serialized into something other than js primitives (boolean string, number) or array, object, null
otherwise there is no guarantee for it to be transported properly over graphql protocol.

Here `DateTime` scalar was being serialized to an instance of `Date` - in contrary to `Date` or `Time` scalars, which have been left untouched in v1.2.0 and still serialize to string - and that broke the expected outputs, mangling data for fractional seconds and failing to comply with graphql spec which does not allow such values.

Moreover, `astFromValue('2021-10-02T00:00:00.000Z', DateTime)` (a function from `graphql/utilities`) fails on such "serialized" value (here: `Date`) with `TypeError: Cannot convert value to AST: 2021-10-02T00:00:00.000Z.` - that was the initial reason for me to submit the PR - as it broke our toolset.

The affecting change happened in Urigo@3b1352c#diff-5bff20d592f8d56ae20cad088bf374d5ce38af414afd5631ab82f42481bb8473
with no message explaining why, no linked PR, moreover there is no release notes for v1.2.0 (https://github.com/Urigo/graphql-scalars/releases/tag/v1.2.0) only v1.2.1 but there is nothing mentioning the change in any of future releases.

Additionally, fixed inconsistencies in handling unix timestamps (maybe that was initial motivation to go with those changes in v1.2.0?)

# Conflicts:
#	src/scalars/iso-date/DateTime.ts
#	tests/iso-date/DateTime.integration.test.ts
#	tests/iso-date/DateTime.test.ts
#	tests/iso-date/formatter.test.ts

graphql-iso-date operated on Unix timestamp values, but graphql-scalars operates on ECMAScript timestamps (number of milliseconds since January 1, 1970, UTC)
as decided in Urigo#387 (comment)

It has to be clear which is used. Certainly values are not Unix timestamps and all references must be removed.
Docs are updated.

ref: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Date#the_ecmascript_epoch_and_timestamps

value of ISO string representation of ECMAScript timestamp

Co-authored-by: Sumegh Ghutke <[email protected]>