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

[droberts195]

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 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.

[edsavage]

As std::chrono::parse will only be available from the C++20 standard (https://en.cppreference.com/w/cpp/chrono/parse), which is not yet supported by our current compiler set, this investigation will be revisited at such a time when they do.