Timestamp from chrono DateTime doesn't follow spec for pre-UNIX epoch dates

[neoeinstein]

The contract for a timestamp requires that a timestamp with a negative seconds component must have a negative nanos component. As currently implemented, the nanos component will always be positive when converting from a chrono DateTime. This may cause issues with downstream consumers who expect that the nanos field will follow the behavior defined in the documentation and require a timestamp from before 1970.

For reference on how to correct this, see the implementation of From<SystemTime> in prost-types. (It would also be nice if pbjson-types included a From<SystemTime> implementation as well.)

[benluelo]

According to the spec, the nanos field is to always be positive:

Non-negative fractions of a second at nanosecond resolution. Negative second values with fractions must still have non-negative nanos values that count forward in time. Must be from 0 to 999,999,999 inclusive.