"And for 2021 he got the remission of 21 days i.e for Jan, Feb and March as he was released on 3rd April 2021"
Why 3rd Apr 2021? Do you mean 3rd Feb 2022?

This is my approach. Correct me if thing went wrong.
My manual calculation: 11/12/2020

Actual start date in jail: 4/5/2018-2*30-29=4/2/2018
Full months in 2018: 10. Remission days = 10 * 7 = 70 days
From 1/1/2019-31/12/2021: full 3 years * 114 = 342 days
From 1/1/2022-3/2/2022: full 1 month * 7 = 7 days
TTL days: 70+342+7=419 days

End date w/o remission: = 4/2/2018+4*365=3/2/2022
Deduct 419 days
=3/2/2022-419=11/12/2020