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The 29th Dimension is a dimension completely seperate from ours. A portal to this dimension opens at 12:01 every February 29th, and promptly closes at 11:59 that evening. Thus, anyone who steps into this portal can only step back through, and go back to their own dimension, every four years.
On February 29, 4012, the Tenth Doctor landed his TARDIS in this dimension, intending to save all of those stuck there.