R aggregate() with dates
I am going to agree with @user1683454's comment. After importing, your DATE column is of either character, or factor class (depending on your settings for stringsAsFactors). Therefore, I think that you can solve this issue in at least several ways, as follows:

1) Convert data to correct type during import. To do this, just use the following options of read.csv(): stringsAsFactors (or as.is) and colClasses. By default, you can specify conversion to Date or POSIXct classes. If you need a non-standard format, you have two options. First, if you have a single Date column, you can use as.Date.character() to pass the desired format to colClasses. Second, if you have multiple Date columns, you can write a function for that and pass it to colClasses via setAs(). Both options are discussed here: https://stackoverflow.com/questions/13022299/specify-date-format-for-colclasses-argument-in-read-table-read-csv.

2) Convert data to correct format after import. Thus, after calling read.csv(), you would have to execute the following code: dateColumn &lt;- as.Date(dateColumn, "%m/%d/%Y") or dateColumn &lt;- strptime(dateColumn, "%m/%d/%Y") (adjust the format to whatever Date format you need).