What's the best practice for getting a random DateTime between two date-times?

You could try using:

var randomTest = new Random();

TimeSpan timeSpan = endDate - startDate;
TimeSpan newSpan = new TimeSpan(0, randomTest.Next(0, (int)timeSpan.TotalMinutes), 0);
DateTime newDate = startDate + newSpan;

This will give you different times down to the minute. If you want 100 (or any thing more than 1) DateTimes then only create the Random object once. The MSDN page on Random explains in detail why creating several Random objects in quick succession is a bad idea.

Using a different TimeSpan constructor will give you different granularity. From the TimeSpan constructor MSDN:

TimeSpan(Int64) Initializes a new TimeSpan to the specified number of ticks.
TimeSpan(Int32, Int32, Int32) Initializes a new TimeSpan to a specified number of hours, minutes, and seconds.
TimeSpan(Int32, Int32, Int32, Int32) Initializes a new TimeSpan to a specified number of days, hours, minutes, and seconds.
TimeSpan(Int32, Int32, Int32, Int32, Int32) Initializes a new TimeSpan to a specified number of days, hours, minutes, seconds, and milliseconds.

Here's my algorithm and code:

• find the difference between the two dates
• for each iteration, create a random number between the two dates

• create a new date between them. Simply add that random number as minutes to the start datetime.  

  Random randNum = new Random();
  
  DateTime minDt = new DateTime(2000,1,1,10,0,0);
  DateTime maxDt = new DateTime(2000,1,1,17,0,0);
  List<DateTime> myDates = new List<DateTime>();
  //Random.Next in .NET is non-inclusive to the upper bound (@NickLarsen)
  int minutesDiff = Convert.ToInt32(maxDt.Subtract(minDt).TotalMinutes+1);
  
  for (int i = 0; i < 100; i++)
  {
     // some random number that's no larger than minutesDiff, no smaller than 1
     int r=   randNum.Next(1, minutesDiff); 
     myDates.Add(minDt.AddMinutes(r));
  }
  
  foreach (DateTime d in myDates)
  {
    Console.WriteLine(string.Format("{0:dd-MMM-yyyy hh:mm}",d));
  }
  

This is what I'm using:

class RandomDates
{
    private Random random = new Random();

    public DateTime Date(DateTime? start = null, DateTime? end = null)
    {
        if (start.HasValue && end.HasValue && start.Value >= end.Value)
            throw new Exception("start date must be less than end date!");

        DateTime min = start ?? DateTime.MinValue;
        DateTime max = end ?? DateTime.MaxValue;

        // for timespan approach see: http://stackoverflow.com/q/1483670/1698987
        TimeSpan timeSpan = max - min;

        // for random long see: http://stackoverflow.com/a/677384/1698987
        byte[] bytes = new byte[8];
        random.NextBytes(bytes);

        long int64 = Math.Abs(BitConverter.ToInt64(bytes, 0)) % timeSpan.Ticks;

        TimeSpan newSpan = new TimeSpan(int64);

        return min + newSpan;
    }
}

I used the approach in the accepted answer but modified it slightly as I had issues with it.