Common ProblemsOnly received 28 days of game time for one-month subscription
Monthly subscriptions allow access to the game for a calendar month. The billing date will not change regardless of how long the month is. You are not charged extra for months that have 31 days and are not charged less for February.
There are seven months that have 31 days and one that has 28, totaling a five day gain over the course of a year (six on a leap year) over using a 30 day game time purchase.
If you prefer to purchase game time in 30 day increments, you may purchase game time without a subscription from the Blizzard Shop.