RTC time synchronization not working (CONFIG_RTC_SYSTOHC)

I have the option CONFIG_RTC_SYSTOHC enabled in my kernel:

Device Drivers  --->
    [*] Real Time Clock  --->
        [*]   Set the RTC time based on NTP synchronization

The help says:

CONFIG_RTC_SYSTOHC:

If you say yes here, the system time (wall clock) will be stored in the RTC
specified by RTC_HCTOSYS_DEVICE approximately every 11 minutes if userspace
reports synchronized NTP status.

However, my hardware clock is not being synced. I have RTC_HCTOSYS_DEVICE set correctly:

CONFIG_RTC_HCTOSYS_DEVICE="rtc0"

The above cited help says, the sync works if "if userspace reports synchronized NTP status". What does that mean? I am using ntpdate to sync my time regularly (every 4 hours) using cron.

How can I tell my system that I am ntp synchronized and that I want my hardware clock to be synced ?