I assume you are using VB .Net for this since you were for your other questions, so here's a quick description of what you could do.
- Read in the whole file into an ArrayList, reading it in one line at a time and adding each line as an element of the array.
- After the file is read into the array, go through each element of the array. For each line extract the date part. (How you do this depends heavily on the format of the date you use and the way the rest of the line looks, if you are using comma-delimited fields for example, a simple string.split should suffice.)
- Convert the date string to a DateTime object using the DateTime.Parse method.
- Compare the newly created DateTime object with a DateTime object representing the oldest entry you want to keep.
- If the date you read in is older than the dates you want to keep, remove that line from the array list
- Continue the last few steps above until you've compared all entries in the file
- When done, re-write your file to disk with only the remaining entries in your ArrayList
I leave converting that into code to you...