Use GDateTime to format dates in GLib demo.

Removing localtime_r allows the demo to be built on Windows.
5 jobs for fix-mingw-build in 33 minutes and 11 seconds (queued for 9 minutes and 24 seconds)
Status Job ID Name Coverage
  Build
passed #32583
build

00:05:10

passed #32593
build_clang_libcpp

00:17:54

passed #32585
build_gcc_49

00:10:01

passed #32586
build_mingw64_fedora

00:13:17

failed #32584
build_clang_libcpp

00:04:38