strftime: Offsets with colons

[philrz]

tl;dr

Zed's strftime function currently can only print offsets without colons (e.g., 2024-08-14T12:12:51-0700). However, RFC 3339 (which ZSON time literals are based on) only support offsets with colons (e.g., 2024-08-14T12:12:51-07:00). So to be consistent we may want to add support for this.

Details

At the time this issue is being opened, Zed is at commit 4360baf.

#5221 has a whole lot more context on this topic. A high-level summary:

• The https://github.com/lestrrat-go/strftime library we use for Zed's strftime doesn't seem in a hurry to get too far beyond the POSIX minimum, and that only provides a %z directive that gives offsets without colons.
• Things like GNU Date and the https://github.com/samsonjs/strftime library we now use in Zui support a %:z directive that adds the colon.
• As a user, it feels odd to me that Zed's own strftime can't construct a valid ZSON time literal. This is not a crisis, however, because Zed's cast to time type from a string timestamp parses the offset ok with or without the colon.

If we take up this work, we might want to try to upstream it to https://github.com/lestrrat-go/strftime. Looking over their prior issues/PRs it seems they have a mixed record of accepting extensions to the base set. But I do feel RFC 3339 makes for a compelling argument.