It appears to be an immutable law of time that the longer that a media franchise runs, the more likely it is that it will contradict its own continuity. That is definitely true for the long-running British series Doctor Who, especially when it comes to the history of the Doctor's arch-enemies, the Daleks. The latest Blu-ray box set of Jon Pertwee's classic third season begins with the 1972 story "Day of the Daleks," which complicates the Dalek timeline even more than before.
Source：https://www.cbr.com/doctor-who-day-of-the-daleks-timeline/
