Review relationship between Datetime and Timezone

[maximelenoir]

A Datetime basically bundles a Timespec and a Timezone together:

pub struct Datetime<'a> {
    tz: &'a Timezone,
    stamp: Timespec,
}

The reference is annoying because it pollutes the type (and everything using it) with a lifetime and by binding the Datetime to its Timezone, making it difficult to move the Datetime around. Other solutions exist to circumvent this issue, although none of them seems to really stand out.

Type	Pros	Cons
reference	Sharable between Datetimes	Makes Datetime difficult to move around; lifetime pollution
owned	Straightforward	Timezone is moved on Datetime creation; would need to clone when creating several Datetimes with the same Timezone
Rc	Easily shared between multiple Datetimes	Ref count overhead
Arc	Easily shared between multiple Datetimes; cross thread boundaries	Ref count overhead

Another solution would be to actually let the user choose the adequate solution through the use of the Borrow trait. Although I haven't dig deep on this solution, I believe it would be awkward to use because of the way the current API is written i.e. Timezone creates Datetime. If the calls were reversed i.e. a Datetime is created from a Timezone and some parameters (e.g. unix timestamp, date, ...) then Borrow would become usable.