generating a new variable based on variables of different dates

[Ji Dummy]

Hello Stata Experts

I`m unable to generate a logic code that would generate 2 new variables
Variable C = A(t) / B(t-1)
Variable D = [D(t-2)-D(t-1)]/D(t-2)
t is date representing years as in the year 2010, 2011,...

I tried lagging variable B by one year but I got some weird outputs
if you have any other suggestions I would really appreciate it

Thank you

[Nick Cox]

Please show example data and the code you tried.