Roshi san, the concept of time seems to be just a mental idea where the passage of time is marked by the processes of the phenomenal world. During zazen time does not seem to exist. What exactly is the nature of time?

Time is being. You cannot divide time and being, that is why what doesn´t have a form, does not have the expression of time. You understand, right? If you sit Zazen, and you still are aware of your body and thoughts, then there is time. When you forget your body, when there are no thoughts, then there is also no time.