ProPublica's Sebastian Jones notes what for us has been one of the most maddening features of campaign finance data: the dates don't mean very much at all. Campaigns report when they receive money, and political action committees report when they donate it. In theory, those dates should match, but as Jones points out, they don't:

The Federal Election Commission [3] requires politicians to disclose the dates and amounts of contributions they receive in periodic reports. Corporate and special interest political action committees (PACs) are also required to file reports with the FEC, disclosing the money they contribute ...