I've read from different sources about the same battles and the dates of some of the battles are not the same. Is this because some sources didn't consider after the battle skirmishing or some considered the day of retreat of the army or something else?
____________________ That old man...had my division massacred at Gettysburg!"
- George Pickett said these words to John S. Mosby shortly after paying Lee a visit in Richmond
"Well, it made you famous"
- Mosby's reply to Pickett
Without knowing which battles you're specifically talking about, I think a number of authors will talk about an entire campaign, rather than just the fighting. Additionally, as you surmise, some take into account preceding actions. Take Chickamauga, for instance. October 19 and 20 are what most would consider the actual battle, but a number of accounts I've read consider the actions on October 18 as part of the fight.
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