Database pricing in the Cloud


I’ve been taking a look at different cloud options and I’m not sure that I understand the pricing structures. For example, in Amazon, one of the databases is $.025 per hour. Is the “per hour” based on time that users are accessing the database (i.e. based on time for queries) or is it calendar based (i.e. for one month, the cost would be approximately .025 * 24 * 31)?