> Have a look at the gcc info pages:
> 
> $ info gcc "invoking gcc" "warning options"

Sure, I'm not stupid! I looked for ~10 minutes how to turn off the remaining 
warnings, but I could not find it. The code is now correct, like I do want the 
"%y" format specifier in the strftime() function, but the warning is wrong. The 
closest I came to was 

-W -Wall -Wno-format

which removes ther warning in strftime(), but I do want this warning, since it 
helps in many other printf() statements.