Check difference in seconds between two times

Assuming dateTime1 and dateTime2 are DateTime values:

var diffInSeconds = (dateTime1 - dateTime2).TotalSeconds;

In your case, you 'd use DateTime.Now as one of the values and the time in the list as the other. Be careful of the order, as the result can be negative if dateTime1 is earlier than dateTime2.

DateTime has a Subtract method and an overloaded - operator for just such an occasion:

DateTime now = DateTime.UtcNow;
TimeSpan difference = now.Subtract(otherTime); // could also write `now - otherTime`
if (difference.TotalSeconds > 5) { ... }

This version always returns the number of seconds difference as a positive number (same result as @freedeveloper's solution):

var seconds = System.Math.Abs((date1 - date2).TotalSeconds);