[HECnet] Another date limit?

In the Phase IV network management spec the description of the encoding of the event log message has this field: JULIAN HALF DAY (2) : B Number of half days since 1 Jan 1977 and before 9 Nov 2021 (0-32767). For example, the morning of Jan 1, 1977 is 0. It's a 16 bit field containing a 15 bit value; there is no explanation why the upper bit is not used even though it's marked as an unsigned integer. As shown, the field overflows in late 2021. If one ignores the arbitrary limit of 15 bits, the end date is 18 September 2066, which is more civilized. I wonder if implementations ignore the 15 bit limit. I think RSTS/E ignores it.