Backport of 5642366f10fa86cc6593f53953cf4d3f346aea8c from the main development trunk. Original commit message:

Fix non-constant-expression narrowing.
For OS X, the microseconds field is implemented as an int type. The implicit narrowing in the initializer list throws a compiler error for some compilers with C++11 support turned on. The specific error message is: "error: non-constant-expression cannot be narrowed from type 'long' to '__darwin_suseconds_t' (aka 'int') in initializer list [-Wc++11-narrowing]".

Tested on Clang 5.1.0 and Mac OS X 10.9.4.