Move parts of `SystemTime` into core?

[jhpratt]

@josh

Crates that just need to process SystemTime instances, but doesn't need to create SystemTime instances, could build without std. For instance, crates that just need to declare trait instances for SystemTime could build without std.

Sure, the crate could build without std, but that serves no purpose unless someone is able to create a SystemTime.

All I can think of is someone having a crate that does operations on it that currently requires a "std" feature flag (there are plenty examples here). This would eliminate the need for it to be gated, while still allowing operations, depending on the internal representation, of course. I personally don't find that to be justification enough, but I suppose that can be debated.

@josh

Also, crates could create a SystemTime by adding a Duration to the EPOCH.

As I said:

@jhpratt

now we just have a convoluted way to add and subtract Durations.

If you're going to construct a SystemTime from the Unix timestamp or similar, would it not make more sense at that point to have a struct with plenty of methods & inspection capabilities, rather than a mostly opaque object?