Roundtrip to/from gpst_nanoseconds are not equal.

[matte1]

Bug report

Roundtrip conversion of an epoch does not result in identical values.

Describe the bug

A clear and concise description of what the bug is.

To Reproduce

        let epoch = &Epoch::from_str("2020-11-15 12:34:56.789 TDB").unwrap();
        let epoch_as_nanoseconds = epoch.to_gpst_nanoseconds().unwrap();
        let epoch_from_nanoseconds = Epoch::from_gpst_nanoseconds(epoch_as_nanoseconds);
        assert!(
            epoch == &epoch_from_nanoseconds,
            "{} {}",
            epoch,
            epoch_from_nanoseconds
        );

Results in: 2020-11-15T12:34:56.789000000 TDB 2020-11-15T12:34:05.606254592 GPST

Expected behavior

A roundtrip using the provided functions should result in identical inputs.

Version

We noticed this happening on switching from Anise 0.4.0 from 0.3.2

[gwbres]

@ChristopherRabotin

I thought all reciprocal cases were tested, is it not entirely ?

[ChristopherRabotin]

I am just as surprised as you are. I have had time to look into this since it was reported a few hours ago, just that I could reproduce that bug when running through the same steps via the FromStr trait. I found a 65 nanosecond difference in the use case I tested.

[ChristopherRabotin]

Thanks for the report, Matt. The crux of the issue is related #229 because the code currently converted the u64 nanoseconds input into a f64 prior to initialization. I'm fixing this right now, and hope to release an updated version at the end of the first week of August. If this timeline is too late for you, let me know, and I can release a patch 4.0.0-alpha before then.

I'd be happy to read your thoughts on the latest comment in #186 which would have prevented this issue entirely.