Added zero property to Duration, TickDuration, and FracSec.

It makes it more pleasant to get those types with a value of zero that
way.

src/core/time.d

@@ -109,6 +109,17 @@ struct Duration
 
 public:
 
+    /++
+        A $(D Duration) of $(D 0). It's shorter than doing something like
+        $(D dur!"seconds"(0)) and more explicit than $(D Duration.init).
+      +/
+    static @property @safe pure nothrow Duration zero() { return Duration(0); }
+
+    unittest
+    {
+        assert(zero == dur!"seconds"(0));
+    }
+
     /++
         Compares this $(D Duration) with the given $(D Duration).
 
@@ -1450,6 +1461,19 @@ struct TickDuration
     static immutable TickDuration appOrigin;
 
 
+    /++
+        It's the same as $(D TickDuration(0)), but it's provided to be
+        consistent with $(D Duration) and $(D FracSec), which provide $(D zero)
+        properties.
+      +/
+    static @property @safe pure nothrow TickDuration zero() { return TickDuration(0); }
+
+    unittest
+    {
+        assert(zero == TickDuration(0));
+    }
+
+
     @trusted shared static this()
     {
         version(Windows)
@@ -2303,6 +2327,18 @@ struct FracSec
 {
 public:
 
+    /++
+        A $(D FracSec) of $(D 0). It's shorter than doing something like
+        $(D FracSec.from!"msecs"(0)) and more explicit than $(D FracSec.init).
+      +/
+    static @property @safe pure nothrow FracSec zero() { return FracSec(0); }
+
+    unittest
+    {
+        assert(zero == FracSec.from!"msecs"(0));
+    }
+
+
     /++
         Create a $(D FracSec) from the given units ($(D "msecs"), $(D "usecs"),
         or $(D "hnsecs")).
@@ -2322,7 +2358,9 @@ public:
            units == "hnsecs" ||
            units == "nsecs")
     {
-        return FracSec(cast(int)convert!(units, "hnsecs")(value));
+        immutable hnsecs = cast(int)convert!(units, "hnsecs")(value);
+        _enforceValid(hnsecs);
+        return FracSec(hnsecs);
     }
 
     unittest
@@ -2368,10 +2406,7 @@ public:
     FracSec opUnary(string op)() @safe const pure nothrow
         if(op == "-")
     {
-        try
-            return FracSec(-_hnsecs);
-        catch(Exception e)
-            assert(0, "FracSec's constructor threw.");
+        return FracSec(-_hnsecs);
     }
 
     unittest
@@ -2425,7 +2460,6 @@ public:
     @property void msecs(int milliseconds) @safe pure
     {
         immutable hnsecs = cast(int)convert!("msecs", "hnsecs")(milliseconds);
-
         _enforceValid(hnsecs);
         _hnsecs = hnsecs;
     }
@@ -2498,7 +2532,6 @@ public:
     @property void usecs(int microseconds) @safe pure
     {
         immutable hnsecs = cast(int)convert!("usecs", "hnsecs")(microseconds);
-
         _enforceValid(hnsecs);
         _hnsecs = hnsecs;
     }
@@ -2651,7 +2684,6 @@ public:
     @property void nsecs(long nsecs) @safe pure
     {
         immutable hnsecs = cast(int)convert!("nsecs", "hnsecs")(nsecs);
-
         _enforceValid(hnsecs);
         _hnsecs = hnsecs;
     }
@@ -2830,8 +2862,7 @@ private:
       +/
     static bool _valid(int hnsecs) @safe pure
     {
-        enum second = convert!("seconds", "hnsecs")(1);
-
+        immutable second = convert!("seconds", "hnsecs")(1);
         return hnsecs > -second && hnsecs < second;
     }
 
@@ -2855,9 +2886,8 @@ private:
             $(D TimeException) if the given hnsecs less than 0 or would result
             in a $(D FracSec) not within the range (-1 second, 1 second).
       +/
-    @safe pure this(int hnsecs)
+    @safe pure nothrow this(int hnsecs)
     {
-        _enforceValid(hnsecs);
         _hnsecs = hnsecs;
     }
 
@@ -3289,7 +3319,7 @@ version(unittest) void _assertThrown(T : Throwable = Exception, E)
     {
         immutable tail = msg.length == 0 ? "." : ": " ~ msg;
 
-        throw new AssertError("assertThrown() failed: No " ~ E.stringof ~ " was thrown" ~ tail, file, line);
+        throw new AssertError("assertThrown() failed: No " ~ T.stringof ~ " was thrown" ~ tail, file, line);
     }
 }