How remove dynamic substring from string in selenium java

You may try a regex string replacement:

String input = "Nov 19, 2019 05:41:08 AM EST";
String output = input.replaceAll("\\b(\\d{2}):\\d{2}:\\d{2}\\b", "$1");
System.out.println(output);

This prints:

Nov 19, 2019 05 AM EST

A perhaps more robust approach would be to go back to the Date, LocalDate/LocalDateTime which generated the current output and instead format using the new mask you want.

You can use regex to remove everything between the first : and the first blank space

String original = "Nov 19, 2019 05:41:08 AM EST"; 
String stripped = original.replaceAll(":.*? ", " ");
System.out.print(stripped); // prints Nov 19, 2019 05 AM EST

If you are not comfortable with RegEx, you can use the following solution:

public class Main {
    public static void main(String[] args) {        
        String dateString="Nov 19, 2019 05:41:08 AM EST";
        String requiredString=dateString.replace(dateString.substring(dateString.indexOf(':'),dateString.indexOf(' ',dateString.indexOf(':'))),"");
        System.out.println(requiredString);
    }
}

Output:

Nov 19, 2019 05 AM EST

There are many other ways as well e.g. using DateTimeFormatter