Duration at runtime

[Zekereth via Digitalmars-d-learn]

I'm confused by the following:

import std.stdio;
import std.datetime;

void main()
{
	string unitType = "seconds";
	auto seconds = 1;
//	auto myDur = dur!(unitType)(seconds); // Error unitType can't 
be read at compile time.
	auto myDur = dur!("seconds")(seconds); // Compiles why?
}

How is seconds able to be read at compile time but unitType 
cannot?

Thanks!