I have a dataset with 3 variables and it looks like:
year diff1 diff2
1970 1 0.5
1980 5 10
1990 6 8
Now I want to subtract the base effect (1970 values) from 1980 and 1990 for variables "diff1" and "diff2".

While I can do this manually, I do not want to hardcode 1970 values.

My code:

replace diff1 = diff1 - 1
replace diff2 = diff1 - 0.5

Is there a way through which I can do this without hardcoding 1970 values?