This log message means that updating the real time clock (RTC) with the time received from the NTP server has failed at that instant. This could be a temporary issue and RTC clock can be updated successfully a bit later. But then the message shouldn't be seen any more. If the message log is persistent, it can mean that there is an issue with the synchronization of RTC clock and time received from NTP server
You can also check the below KB which describes the issue and workaround
Can you check if you are getting the correct date and year when you run the command: "show system uptime".
If you getting wrong year or date, then follow the below process:
Upgrade Junos OS to 15.1X53-D56 or Check recommended release in Juniper.net
>Temporary workaround for Junos OS 15.1X53-D50
Set the initial date to the current date from the CLI.
rtc8564je_rtc0: RTC ERROR(16): read failed for off:2(len:1) rtc8564je_rtc0: SETTIME failed for seconds: error 16 warning: clock_settime failed (16), time-of-day clock not adjusted to system time
Note:After rebooting, the system falls back to the time displayed as 2038. The permanent solution is to upgrade to Junos OS 15.1X53-D56 or higher Even upgrade fails due to this RTC error so please "set date " manually and then upgrade. For upgrade space constraints refer KB31198