Add support for parsing datetimes from strings

[mathrick]

Description

Currently, the only way to create a datetime is to use now() or the datetime constructor with individual components explicitly provided. However, that is essentially never useful for ingesting external inputs, meaning that for any data coming in from JSON, YAML, etc., the user has to hack in a string parser themselves to be able to use time manipulation functions in any capacity. That's a big drawback for things like invoice templates.

There should be a way to parse a datetime-containing string into a datetime that's built-in. There was a previous PR in #1668 which aimed to implement parsing Unix timestamps, but it got closed in favour of a generalised implementation that could parse more formats. It doesn't seem that that implementation ever materialised, nor does there seem to be an existing discussion of it. I'm opening this issue to hopefully revive the topic.

Use Case

Any situation in which external data are ingested and contain dates that need to be manipulated. For example:

• Generating invoices with due dates automatically calculated, based on the contents of a JSON file
• Displaying a date value in a different format than the one given (i.e. "2024-07-01" → "1 July 2024")
• Extracting a date to be displayed out of a a full timestamp given as a string
• Generating a calendar for a given month, based on a value passed in through sys.inputs

[istudyatuni]

Yaml support datetime. You can even do something like

#let date = yaml.decode("date: 2024-07-01").date

[mathrick]

Not inside typst, that is actually a string. Try this:

#let date = yaml.decode("date: 2024-07-01").date
#type(date)

I don't believe YAML supports timestamps as much as it allows applications to recognise certain values and convert them to timestamps automatically. Either way, that does nothing for other formats such as JSON or sys.inputs, so the need for a parser is still there.

[istudyatuni]

I was wrong, it works for toml, not yaml, so you can define function

#let parse(s) = toml.decode("date = " + s).date

[mathrick]

I was wrong, it works for toml, not yaml, so you can define function

#let parse(s) = toml.decode("date = " + s).date

That's a neat hack! Though I still think it should be promoted to properly supported functionality :)