Applying the extent transform to a field that contains Date instances returns an array of timestamps

Applying the extent transform to a field that contains Date instances returns an array of timestamps rather than an array of Date instances.

To recreate, paste the following specification into the Vega Editor. Notice that the value of the dateDomain signal is an array of timestamps rather than an array of Date instances, even though the date field contains Date instances.

{
  "$schema": "https://vega.github.io/schema/vega/v5.json",
  "data": [
    {
      "name": "table",
      "transform": [
        {"type": "sequence", "start": 1, "stop": 8, "as": "index"},
        {
          "type": "formula",
          "expr": "datetime(2021, 0, datum.index)",
          "as": "date"
        },
        {"type": "formula", "expr": "time(datum.date)", "as": "timestamp"},
        {"type": "extent", "field": "date", "signal": "dateDomain"}
      ]
    }
  ]
}

I’ve raised this bug because I think that the extent of a field should have the same type as the field: one would expect a field of numbers to have an extent that was also numbers, for example. However, I appreciate that Date instances can be problematic and that returning timestamps may be preferable. If this is a feature rather than a bug, then I will open a PR on the documentation.

[jheer]

Yes, or just +date suffices for date-to-number coercion in most situations, assuming no null values are involved.

[domoritz]

You are right, Vega does not transparently handle timestamps/datetime in custom expressions.