Do digital multimeters require calibration?
For hobby/student DMMs, the answer is no. You don't have to calibrate it every year. Please take note of the quote: "A long calibration period for the digital multimeter is normally to be advised, except when particularly demanding testing is required.". For a 3 1/2 digit battery-powered DMM, most never get calibrated after being bought.

If you're using a 6 1/2 digit unit, and measuring microvolts to trouble-shoot medical equipment, that's another story.

It all comes down to how important absolute accuracy is to you.

However, your belief that "only analog multimeter require calibration" is dead wrong. The distinction between analog and digital in this case only applies to the display. An analog multimeter uses its conditioning / amplifier circuits to directly drive an analog meter. A digital unit uses its conditioning / amplifier to drive an A/D converter. In both cases, if the analog circuits get out of whack, the meter will give bad results. A problem is more likely to be noticed in a digital meter simply because DMMs make small errors easier to see.