CDO

Dear CDO users,
I have files with the period 1850-2100. In these files, one time period represents the historical (1850-2014) data, and another period represents a different future scenario (2015-2100) data such as ssp126, ssp245. So I merge the time of historical periods with future scenario files. Now I have a file with the same historical period along with different future scenario files. e.g. His_ssp126_1850-2100 and His_ssp245_1850-2100.nc

Now I want to do the running mean with 5 years, 10 years, and 30 years. I know this can be done via e.g.

cdo runmean,30 historical_ssp126.nc  new_historical_ssp126.nc
cdo runmean,30 historical_ssp245.nc new_historical_ssp245.nc

But my issue is that after doing the running mean, the starting year for my future period also changed. For example, If I run the above code for the file (1850-2100), I got my initial year for the future scenario (ssp126, ssp245) is 2000. But I want the year 2015 for the initial year after 5,10, and 30-year running mean.

So, I am getting confused about which period I should select so that if I do the running mean code, then my starting year for the future would be 2015.

Two files are attached.

Thank You
Kunal