Non-linear time series analysis

I´m looking for help to anlyse in R (or SAS) a non-linear time series dataset. I worked with six cows during three days and registered urine events of each cow during each day. I have 4 to 12 events for each cow and day that are non-equally spaced in time. I want to know if I have an statistically trend on their behavior with respect to urination along the day. Thanks!