Improve the available precision for NSDate to Instant conversion

[dkhalanskyjb]

Turns out, a roundtrip in the near times only loses 0.5 microseconds of precision, whereas at the far ends of the supported range, we lose 10 microseconds. It is a lot, but much less than the microsecond precision loss that happens when an NSDate is formatted (which led us to this implementation in the first place).

Fixes #427