UTC timeunits do not return UTC times in Vega-Lite 4.0

In Vega-Lite 4.0, UTC timeunits return the same value as their non-UTC counterparts. This was not the case in Vega-Lite 3.X

Here is a simple spec that demonstrates this:

{
  "data": {"values": [{"t": "2020-01-01T12:00:00-08:00"}]},
  "mark": "point",
  "encoding": {
    "x": {"type": "temporal", "field": "t", "timeUnit": "hours", "title": "hours"},
    "y": {"type": "temporal", "field": "t", "timeUnit": "utchours", "title": "utchours"}
  }
}

I viewed this on a computer set to the US/Pacific timezone.

With Vega-Lite 3.4.0 & Vega 5.9.1, I see the expected result: the local hour is 12, and the UTC hour is 20.

With Vega-Lite 4.0.2 & Vega 5.9.1, the result has changed: the UTC timeUnit is the same as the local timeUnit:

This seems to be the case not just for utchours but for every UTC timeUnit Vega-Lite supports: each UTC timeUnit returns identical timestamps as its non-UTC counterpart.

[haldenl]

@SyntaxRules It looks like the issue has to do with the inconsistent parsing of the Date object (see here). In the spec, data is given in year-month-day format, which is interpreted in UTC. The datetime objects provided as tick values are interpreted in local time. If we switch to the same formatting:

{
  "$schema": "https://vega.github.io/schema/vega-lite/v4.json",
  "data": {
    "values": [
      {"d": "2020-01-01"},
      {"d": "2020-01-15"},
      {"d": "2020-02-01"},
      {"d": "2020-02-15"},
      {"d": "2020-03-01"}
    ]
  },
  "mark": "rule",
  "width": 600,
  "height": 100,
  "encoding": {
    "x": {
      "scale": {"domain": ["2019-12-15", "2020-04-01"]},
      "axis": {
        "format": "%Y-%m-%d",
        "orient": "bottom",
        "title": "CY",
        "values": ["2019-12-01", "2020-01-01", "2020-02-01", "2020-03-01"]
      },
      "field": "d",
      "type": "temporal",
      "timeUnit": "utcyearmonthdate"
    }
  }
}

The output is as expected (on the fix branch). I wonder if something changed with datetime usage between 3.2.1 and 4.0. Will investigate.

[domoritz]

@SyntaxRules After the fix

This seems right since the dates are parsed as local, right?