Mean = 27.6, lately 24, is there a signifcant difference?

#1
Hello!

I am having a major brain issues today, and have a question that I believe many people will be able to help me with. I have a task I complete every month, and have data for the past 64 months. Over that time, the average time it takes to complete the task is 27.6 days, with a standard deviation of 2.618 days. The last few months (4), it has taken 24 days. Is that significantly different then 27.6 days?

In my attempt, I calculated a Z score, and using a normal distribution calculated what I believe to be a p value of 1.89e-28. This is where I am having my issues? Does that tell me that there is a significant difference? Or have I royally messed up?

Any help would be greatly appreciated!

Peace.Love.Math