do you have a hard time remembering dates?

Getting them into my mind yes, but when it's there I can remember it for years. When I stop using it for a certain amount of years though I'll tend to forget it.

I am usually quite good at remembering things, and to make it easier I usually write birthday's and anniversaries down, then I should never forget them.

Not at all. As a matter of fact, once I get the birth date of someone, it sticks in my memory and hardly ever leaves unless I lose contact with the person for a very long time.

only remember about how many time left for montly salary and monthly billing sometimes forgot about someone anniversaries

It is easy to remember birthdays of significant people like your family and the important dates that relates to you like date of graduation. It is also easy to remember dates of special holidays( that one that have red marks on the calendar). Other than those it is pretty hard to remember dates.