There are 5 steps in evaluating a patient. Each patient needs evaluation. Sometimes the routine is slightly different for each case. Recording the time is a way of signalling when the evaluation began.
Also it is a good idea to check a clock when you are finished. It shows that the time between beginning and end was enough time to conduct a thorough investigation or thorough enough.
I'm going to check this with a nurse I'll be speaking to this afternoon. If there are differences in the answer, I'll edit my answer.