1. A method for protecting backup data stored in a data backup system, the backup data being created in accordance with a data backup policy, the method comprising: defining a plurality of time windows for creating and maintaining backup data in accordance with the data backup policy; assigning each of the time windows a predetermined amount of storage space; creating backup data in accordance with the data backup policy; determining whether a storage space assigned to a particular time window is large enough for new backup data; storing the new backup data at the particular time window if the storage space is large enough to store the new backup data, and if the storage space is not large enough to store the new backup data at the particular time window, then deleting the oldest backup data until enough storage space for the new backup data in the assigned storage space at the particular time window is obtained; and deleting the new backup data subsequent to retaining the new backup data in the data backup system for a specific time duration, wherein the specific time duration is indicated by the data backup policy and is calculated using an internal clock that is independent from a main system clock of the data backup system, wherein the internal clock is located in a data protection unit of the data backup system and the main system clock is located in a host computer of the data backup system, wherein the internal clock calculates the specific time duration by counting time during which the data backup system is operative, and wherein the internal clock is not functional and does not count time during which the data backup system is not operative.