rule 119 a and b

it is like that, 119 a, part / fraction of the month is to be ignored, if interest has to be calculated on annual basis for ex delay in period is 7 months and 21 days, number of months to be taken as 7.
reg . 119 b, fraction of a month to be considered as full month , in case interest calculated on monthly basis for ex delay 4 months 2 days, then number of months to be taken is 5..
For calculating interest amount to be rounded off to nearest multiple of RS. 100 for example actual amount is 5425 = 5400 and 5451 = 5400 only.

any interest or int us 234 a b c

yes, same method of monthly calculation, as 234 a b c are related to every month delay

for example int is 5490 then 5400 or 5500

5400 only
fraction of rupees over multiples of 100 to be ignored