Provide construction and/or parsing for ZoneOffset

[cromefire]

Currently you are not able to directly parse strings like +02:00 into an ZoneOffset. You can do so via

TimeZone.of("+02:00").offsetAt(Instant.fromEpochMilliseconds(0))

but it's rather a workaround, than a real solution (and if that's important probably less performant/efficient/discoverable).

My Idea would be to provide an ofHoursMinutes() (and maybe also ofHours(), ofHoursMinutesSeconds() and ofTotalSeconds​()) function similar to Java:

fun ofHoursMinutes(hours: Int, minutes: Int): ZoneOffset

and a parsing function that parses the time according to RFC3339 time-numoffset or time-offset (like Java's of()):

fun of(offsetId: String): ZoneOffset

ZoneOffset.of("+02:00")
ZoneOffset.of("Z")

A ZoneOffset.UTC would also be great.

This would greatly simplify #90.

edited to adjust my ideas more towards what Java already does

[cromefire]

It's also probably doable in common code, so implementing it once should be sufficient (although for Java you could pretty much just delegate most things)

[miguelhrocha]

Hey @cromefire do you still need someone to look into this? I can work on that this week

[cromefire]

Yes I still think that's a good addition, because I mean if you can't parse a zone offset into a ZoneOffset that doesn't seem very intuitive, right?

[miguelhrocha]

I think it makes total sense, it'll also help Java devs wanting to adopt the library. I'll work on it 👍

[dkhalanskyjb]

Adding a way to parse ZoneOffset would not be a big change—more or less all the required code is in place, we only need to make it public under some name—but we plan to change the role that ZoneOffset plays in our library. Until then, we intentionally keep the API surface of ZoneOffset slim.

As a side note, with the latest release, it is also possible to parse a ZoneOffset like this:

TimeZone.of("+02:00") as ZoneOffset

[ilya-g]

In the course of ZoneOffset refactoring (see #125) we're introducing a new UtcOffset class instead of ZoneOffset which can be constructed in various ways:

• parsed from an ISO string, e.g +02:00
• from total number of seconds: UtcOffset(seconds = 1800)
• from total number of minutes and number of seconds within a minute: UtcOffset(minutes = 90, seconds = 0)
• from hours, minutes, seconds components: UtcOffset(hours = 3, minutes = 30)