I've implemented a timer for situations like this before: I actually ended up with a class with two different implemations, one for Windows and one for POSIX.
The reason was that Windows has the QueryPerformanceCounter() function which gives you access to a very accurate clock which is ideal for such timings.
On POSIX however this isn't available so I just used boost.datetime's classes to store the start and end times then calculated the duration from those. It offers a "high resolution" timer but the resolution is undefined and varies from platform to platform.
