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Q1: Isn't this article incorrect where it says Armstrong and Aldrin "stayed a total of about 21½ hours on the lunar surface"? (No.)
They landed their spacecraft on July 20, 1969, at 20:18 UTC, and took off again the next day at 17:54 UTC. Therefore, they spent a total of 21 hours and 36 minutes on the surface, most of this time inside the spacecraft. Their moonwalk outside the spacecraft lasted just over 2 hours and 31 minutes. Both facts are recorded accurately in the summary.
Q2: The date for the Moon walk is wrong; it should be July 20. (No; it was July 21, UTC)
The dates of the Apollo 11 Moon landing and Moon walk are often reported as July 20, 1969, using time zones of the United States. However, the Moon is not within any Earth-bound time zone, and to avoid regional bias, the WP:WikiProject Spaceflight community has established a consensus (discussed here) to use UTC. The date and times of the landing and EVA were July 20, 1969, 20:18 UTC and July 21 at 02:56:15 UTC, respectively. Therefore, we use July 20 as the date of the landing and July 21 as the date of the EVA. Please don't "correct" the EVA date to July 20.