How long should "1 month" be?

If in my project someone buys a membership for 1 month, how long should that "month" be?

• Always 30 days, no matter the month of the year (e.g 23/2/2016 => 25/3/2016)

• The same day number of the next month (e.g 10/5/2016 => 10/6/2016)

• Or another?