OK, so I made another test and found out, that the clock runs exactly 15 seconds ahead in 24 hours, which means 1 minute every 4 days which brings it to 15minutes every 2 months.
I also contacted Futaba service and they said, that clocks in these radios were never accurate, but 7,5 minutes per month is alot and they never heard of it beeing that much off.
Solution would be to replace the PCB.
I'm really angry, since I wanted to return the radio to the seller, but the seller refused to take it back after little over 1 month, since their return policy was 14 days.
And the radios is brand new, never used, since it's winter now.