ExpenseConnect (EC) uses the date of the last fully successful batch (aka all reports are successful) as the "start date" to look for reports. So if you keep importing batches that are not fully successful (e.g. some reports error out), EC will continue pulling all of the same reports in order to keep trying to pull the unsuccessful ones.
If you would like skip the errant reports in order to get a fully successful batch, you can trick EC into thinking they are successful by checking the success checkbox next to them in the Batches form.
Once you get a fully successful batch, it will use the date of that batch as the new start date. So during the next batch, it will only pull in reports added after that date.
That is why it is important to check off errant reports as successful so that eventually EC will import a fully successful batch for you.
You can also skip over old reports by changing the start date in the Settings form to the last date you ran an EC batch.