55 days interest free period depends on your billing/statement date and payment deadline date.
For example; if your billing date/statement date of the card is 5th of every month and payment deadline is 30th of every month; consider 5th March as example and if you purchase something on 6th March, it will be shown in your April statement, i.e. on 5th April so that you need to pay it before 30th April only.
So that gives you a 55 day interest free period- from 6th March to 30th April...