Converts a UNIX timestamp in nanoseconds to an NTP time.
The caller should pass a value with nanoseconds since 1970. The NTP time will, in the upper 32 bits, contain the number of seconds since 1900 and, in the lower 32 bits, the fractional seconds. The resulting value can be used as an ntptime for constructing SR RTCP packets.
an UNIX timestamp in nanoseconds
the NTP time for