How to count number of dates in my textfile?

Read the file in:

string[] lines = File.ReadAllLines(path);

Process each line:

foreach (string line in lines)
   {
   ...
   }

If the line contains a date, count it:

int count = 0;
...
foreach (string line in lines)
   {
   ...
   count++;
   }

Put the result to a label control:

myLabel.text = "Number of dates = " + count.ToString();

Working out if it contains a date is up to you: I do not know anything about your data format...

Use regular expression to find the matching date. Loop it till no match occurs.

Here is an example

http://www.java2s.com/Code/CSharp/Development-Class/Useregulartoverifyadate.htm[^]