Do you have a regular time series (a measurement each day) or do you have data only on days with precipitation (date + amount)?

Thank Jochen Wilhelm for your answer. I have daily grid data (for each 0.25 degree (date +amount)).

It seems that you likely read in netCDF files, ege.from https://psl.noaa.gov/data/gridded/data.unified.daily.conus.html.

There are many tutorials and videos you can google on how to access netCDF data via R. I find this one nice: https://cran.r-project.org/web/packages/futureheatwaves/vignettes/starting_from_netcdf.html

Once you have the data in R, then the calculations boil down to applying some "moving window"or "sliding window" function on a numeric vector.

In R you can use filter() for this purpose:

R5D