How to calculate Average of daily data to fill missing values

Hello all,
I have a time series data of daily precipitation for 25 years. These data have huge number of missing values. I want to reconstruct the missing values by calculating the mean of rainfall for before and after that specific date. For example the rainfall for date 1/1/1990 is missing. I want to calculate the average of all rainfall for dates 1/1/1985, 1/1/1986,…, 1/1/1989, 1/1/1991,…,1/1/2010 except the missing one. My question is how to write an equation in excel to do the operation fast. The attached file is a sample of work to be reconstructed. The red column is the SUM and MEAN of all data I calculated them manually but I need a faster way. Any help highly appreciated.
Thank you in advance.