The 25-year-old defender is expected to be in the squad for Saturday's visit to promotion rivals Bamber Bridge.
Bucks' manager Steve Cunningham knows Dan well after a spell together at Curzon Ashton.
Cunningham said: "Dan is well over six feet tall and athletic, strong in the air and quick on the ground. What's more, he can fit in anywhere across the back line.
"Thanks to the chairman and board for their proactive support in getting this deal done. Dan's experience will be a big asset to us, particularly as we struggle with injuries to key players."
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Cowan completed his time as a trainee at Macclesfield Town and went on to play 50 times for the Silkmen, then enjoyed three successful years at Stockport County, before moving to Curzon Ashton, Guiseley and Chester.