Converts an NTP time to UNIX nanoseconds. ntptime can typically be the NTP time of an SR RTCP message and contains, in the
upper 32 bits, the number of seconds since 1900 and, in the lower 32 bits, the fractional seconds. The resulting value will be the number
of nanoseconds since 1970.
| ntptime | an NTP timestamp |
the UNIX time for ntptime in nanoseconds. |