Hi,
I do not know if I understood your question, but I think that your end year could be the last year in your catalogue, maybe 2020. I understand the completeness time interval for each row like the difference between the first and the second year, for example, 2005-1975 = 30 years, that it is the minimum time lapse in that you consider complete your catalogue for earthquake between 3.5 and 3.9 Mw.
I hope it can be helpful.
Cristian.