shell script set the default numbers as double digit zero as prefix

In a shell script, I'm processing, some addition process will print an output. If it is a single-digit one, then it has to add zero as a prefix. Here is my current script:

c_year=2020
for i in {01..11}
		do  
			n_year=2020
			echo 'Next Year:'$n_year
			if [[ $i == 11 ]]
			then n_month=12
			echo 'Next Month:'$n_month
			else 
			n_month=$(($i + 1))
			echo 'Next Month:'$n_month
			fi
			echo $date" : Processing data '$c_year-$i-01 00:00:00' and '$n_year-$n_month-01 00:00:00'"
		done

The i value is in doule digit, but the n_month is still printing single digit. How do I set default shell output should return as double digit? Or any alternate way to solve this?