Investigate replacing strptime() with std::chrono::parse

Currently we are using the `strptime()` C function for time parsing. This is a pain, as it requires workaround for Linux and Windows, so is one of our least platform independent pieces of code.

In theory [`std::chrono::parse`](https://en.cppreference.com/w/cpp/chrono/parse) might be better. However some investigation is required to find out if `std::chrono::parse` is as portable as it should be. Given what we found with `strptime()` Linux and Windows are the platforms most likely to not implement the standard fully, with timezone support being the likely problematic area.

If the portability _is_ better then we should remove our `strptime()` wrapper code and all uses of it and replace them with `std::chrono::parse`. If workarounds would still be required then we might as well stick with what we've got until the C++ library _is_ consistent.