Can't create folder using DateTime as name

Question

I am trying to create a folder using a TimeStamp as the folder name. The code I am using will not create the folder when I use the timeStamp variable in the code below. However if I set the folder name directly like this ...

File dir = new File("Hello") 

The folder is created. Is this the right way to set a folder name using Date and time?

public void logEmData(String reason,Campus c ) throws IOException 
{

    LocalDateTime time = LocalDateTime.now();

    try(FileWriter writer = new FileWriter(file, true))
    {
        writer.write("Building " + c.getName() + " Entered Emergency Mode" + System.lineSeparator());

        writer.write(" Reason: " + reason + System.lineSeparator());

        writer.write(time.toString() + System.lineSeparator());

       //Create folder 
        String timeStamp = "EM_" + time;
        File dir = new File(timeStamp);
        dir.mkdir();

    }

Answer 1

LocalDateTime.now() gets you a date with a format roughly like so: 2015-01-14T16:00:49.455.

: is a problematic character for folder creation under Windows due to it being a reserved character. You may want to consider formatting the string to change it to -.