Switching the 74HCT4060 to a 74HC4060 hasn't solved the problem .
Maybe it's the signal rising and falling time that's too long .
On my scope I would say that it's between 0.4 and 0.6 us .
Anyway, it's not very accurate as, I get this measure with the fastest sampling rate without averaging of my scope , and the two vertical lines that mark the beginning and end of the rise are only 2 pixels far one from each other .
Maybe there's some way to get the signal steeper , with some schmitt trigger or something else ?