public final Class Duration

A Duration represents a signed, fixed-length span of time represented
as a count of seconds and fractions of seconds at nanosecond
resolution. It is independent of any calendar and concepts like "day"
or "month". It is related to Timestamp in that the difference between
two Timestamp values is a Duration and it can be added or subtracted
from a Timestamp. Range is approximately +-10,000 years.
# Examples
Example 1: Compute Duration from two Timestamps in pseudo code.
    Timestamp start = ...;
    Timestamp end = ...;
    Duration duration = ...;
    duration.seconds = end.seconds - start.seconds;
    duration.nanos = end.nanos - start.nanos;
    if (duration.seconds < 0 && duration.nanos > 0) {
      duration.seconds += 1;
      duration.nanos -= 1000000000;
    } else if (durations.seconds > 0 && duration.nanos < 0) {
      duration.seconds -= 1;
      duration.nanos += 1000000000;
    }
Example 2: Compute Timestamp from Timestamp + Duration in pseudo code.
    Timestamp start = ...;
    Duration duration = ...;
    Timestamp end = ...;
    end.seconds = start.seconds + duration.seconds;
    end.nanos = start.nanos + duration.nanos;
    if (end.nanos < 0) {
      end.seconds -= 1;
      end.nanos += 1000000000;
    } else if (end.nanos >= 1000000000) {
      end.seconds += 1;
      end.nanos -= 1000000000;
    }
Example 3: Compute Duration from datetime.timedelta in Python.
    td = datetime.timedelta(days=3, minutes=10)
    duration = Duration()
    duration.FromTimedelta(td)
# JSON Mapping
In JSON format, the Duration type is encoded as a string rather than an
object, where the string ends in the suffix "s" (indicating seconds) and
is preceded by the number of seconds, with nanoseconds expressed as
fractional seconds. For example, 3 seconds with 0 nanoseconds should be
encoded in JSON format as "3s", while 3 seconds and 1 nanosecond should
be expressed in JSON format as "3.000000001s", and 3 seconds and 1
microsecond should be expressed in JSON format as "3.000001s".