Add a DateTime32 type for 32-bit serialized times

[teor2345]

Motivation

In #2178, we want to modify the last seen times gossiped by peers.

But the chrono::DateTime<Utc> type that we're using has a ~56 bit range, so we can easily generate times that don't serialize when we go to send them to peers.

This breaks one of Zebra's design principles: invalid data should be unrepresentable.

Solution

Implement part of #2171:

• add a DateTime32 type
• use it for MetaAddr.last_seen

The code in this pull request has:

• Documentation Comments
• Existing Unit Tests and Property Tests

Review

@jvff can review this PR, it helps simplify #2178.

Follow Up Work

Replace Zebra's other 32-bit times with DateTime32 #2211

• these times are lower risk, because we don't modify or generate them yet

[jvff]

This looks great! I added two minor suggestions, but they are optional.

One question though, would it make sense to also add Add and Sub implementations? I can do that in a separate PR if you want 👍

[dconnolly]

Made some suggestions per @jvff's comment but otherwise looking good 😎

[teor2345]

One question though, would it make sense to also add Add and Sub implementations? I can do that in a separate PR if you want 👍

I'd prefer to do the calculations in chrono::DateTime (~56 bits) or i64. Then we don't have to worry about overflow until we convert back to DateTime32.

Also the chrono::DateTime API is a lot nicer, it has functions for minutes, hours, etc.

[teor2345]

Looks like Deirdre is working on other things right now, so I went ahead and merged this PR so we can make progress.