Checking if two dates are within 15 minutes of each other
Naming:

Method names should contain a verb and describe exactly what they are doing. For example, look at DateComparison. That sounds more like a type name to me, and I have no idea what the method will do by its name alone. 

StripMilliseconds and GetTotalSeconds are much better.



I'd suggest this is as an alternate solution:

public static class TokenValidator
{
    private const int aliveDurationMinutes = 15;

    /// &lt;summary&gt;
    /// Checks if the token is still alive with a 1-second tolerance.
    /// &lt;/summary&gt;
    public static bool CheckTokenAlive(DateTime created, DateTime timeToCompare)
    {
        TimeSpan elapsedTime = timeToCompare - created;

        return (elapsedTime.TotalSeconds &gt; -1 &amp;&amp; elapsedTime.TotalSeconds &lt; 0) // up to 1 second before
               || (elapsedTime.TotalSeconds &gt;= 0 &amp;&amp; Math.Floor(elapsedTime.TotalSeconds) &lt;= aliveDurationMinutes * 60); // up to 15 minutes later
    }
}


I use Math.Floor on the TotalSeconds to add the 1-second tolerance. You can be 0.9999 of a second over and still evaluate as true. You could have done a strictly less than with ((aliveDurationMinutes * 60) + 1) instead.