The short answer is they are not the same.
There is no internationally agreed upon method for specifying length of stay allowed for visa free or visa required entries. Each country has the right to specify their own rules. Most countries use either XX days or X months, but they are also allowed to use XX weeks or even XX hours (as transit visas often do).
Likewise there is no universal method of counting those time periods. Many countries use a "calendar day" (midnight to midnight), so 01:15 or 23:45 both count as one calendar day even though one affords almost 24 hours more in the country. Other countries use a "24 hour period", so if you arrive at 14:00, you need to exit by 13:59 on your last day. To compound things further, some countries using the calendar day method, start their count from the first full day in country, not the partial arrival day.
To answer the OP's question, because of the variances, there is no simple formula to apply. You need to research how each country counts their days, then do the math from there. And you need to use the units specified, you can not assume 30 days and one month are the same.