Thanks for your time bebo021999, I am giving you an example for a guy who was convicted for 4 years. He was convicted on 04-05-2018. The undertrial period was for 2 Months and 29 days.
Now as he was convicted for 4 years on 04-05-2018 his release date without remission would have been 03-05-2022 i.e exactly 4 years form his date of conviction.
Now after he gets convicted he is given the setoff period for his undertrial days, in this case it was 2 Months and 29 Days. So we minus it from his exact date of release i.e 03-05-2022 that comes to 03-02-2022. Form here we start to minus his days months and years from the back date till the tally comes to zero.
Now in this case as he was convicted on 04-05-2018 we deducted 28 days from the 03-02-2022.
Now he will start getting a remission of 7 days per month.
As the criteria for remission is 7 days for the entire month completed i.e 1st till the last date of his month.
Now for any calendar year completed by the prisoner i.e from Jan to Dec he gets 30 days additional remission.
So in this case for the year 2018 he only earned remission of 49 days. i.e 7 months *7
For 2019 he got remission of 84 i.e 7*12 days plus 30 days as he completed the calendar year so the total was of 114 days.
For 2020 he got the same remission of 114 days
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
Remission
2018 2019 2020 2021 Total
49 114 114 21 298

In short we calculate the above table manually and than do the manual calculation
Now his date for release would have been 03-05-2022 as he was convicted for 4 years on 04-05-2018.
After we minus the under trail days of the prisoner i.e 2 months and 29 days we get the date as 02-02-2022.
We apply the manual calculation as 2 months minus 03-05-2022 that comes to 03-03-2022 now we need to minus 29 days that comes to 02-02-2022 as the month of Feb in 2022 was of 28 days.
Now we minus his remission of 298 days so we minus 9 months i.e 270 days and the remaining 28 days from the date we got after deducting his under trial day. That is to minus 9 months from 02-02-2022 so we get the date as 02-05-2022.
Now we deduct 28 days from the 02-05-2021 we get the date as 04th April 2021.