Allow a timezone configuration for partial date handling

Currently, if I set a field to the date type and enter a partial date, e.g., 2021-10-21 I’m given a date object initialized with a value in UTC, like this 2021-10-21T00:00:00.000Z. Ideally I’d be able to have that partial date interpreted correctly for my timezone — I’m in America/New_York so I’d like to see 2021-10-21T04:00:00.000Z.

A possible, straightforward solution would be the date-fns-tz libraries zonedTimeToUtc function and an optional IANA timezone in the contentlayer config. E.g.

zonedTimeToUtc(doc.createdAt, opts.timezone)

If you’re interested in a contribution I could work on a PR.

[mshick]

Apologies for the long delay in testing this, but I was trying out the feature today and noticed a couple things:

Minor issue

The makeSource fn is being exported without the correct types for this feature. It looks like the PartialArgs type in core was updated, but makeSource is exported with a custom Args type passed to the generic.

Larger issue

The utility of this option doesn’t work as I would have imagined. In my custom solution for this issue I treated my field publishedAt as a string, and passed that string to zonedTimeToUtc. This has the result of interpreting the string value in the specific timezone, and then doing the timezone offset. So, the partial date string is converted from 2020-10-20 to 2021-10-20T04:00:00.000Z.

The option not working as expected is due to this line: https://github.com/contentlayerdev/contentlayer/blob/589aa04099f6d096064cb699b5af95a771e75e05/packages/%40contentlayer/source-files/src/fetchData/mapping.ts#L244

If that date object were created only if we don’t need to use zonedTimeToUtc then things should work correctly assuming I specify the frontmatter field in my markdown as an explicit string.

  publishedAt: '2020-10-20'

If I add the field without the quotes though, I think gray-matter is turning it into a Date object before you touch it. I’m not sure if it’s even an option, but since you apply typing explicitly in contentlayer it might make sense to have gray-matter treat the field as a string no matter what, so that you can control when / how it is turned into a Date, and can offer a consistent user experience for the new timezone option — meaning, they can get the TZ applied whether they specify the partial date as a quoted string or an unquoted date-string.

[schickling]

This should be addressed with the 0.2.7 release. 🎉