You can add transactions received in the past to a constituents timeline so you have a more complete picture of their giving - to ensure the transaction appears in correct place on the timeline and the dashboard stats are correct you should set the dates on the transaction.  

To do this add the transaction in the normal way and press the small arrows - as highlighted below - to reveal some further fields - and enter the correct dates in the highlighted fields.

keywords: back date old transactions historical historical transactions old donations historical historical donations