I think you might be confusing objects. The timer is used for intervals which is set via the Interval Property
This property is in milliseconds so if you want to set it using seconds just multiply your seconds value by 1000.
If it is the stopwatch you are asking about then just use the ElapsedTime which is a time span and access the total seconds (as katananesei has said).
System.Diagnostics.Stopwatch sw = new System.Diagnostics.Stopwatch();
var seconds = sw.Elapsed.TotalSeconds;