Baltimore MD (Maryland) Feb 23 5¢ rate. That was the rate for under 300 miles from July 1 1845 to July 1 1851. So unless there is a letter inside that's dated to narrow it down, that would give you a range of when it could have been posted. I believe the distance these days by car is about 150 miles.
Thank you for the very interesting information. Unfortunately there is no letter inside to know the exactly date. Someone has any idea of value of this cancels? This Blue 5 cents appear in the begging of the Scott Catalog (1845). There is any cancel site or catalog about these cancels?
It will give you lots of info for all sorts of stampless covers. I will say that this is a fairly common type - I've pulled the same exact type of postmarks from Baltimore out of dealer's dollar boxes.