made datetime private inline because of preprocessors
made datetime private inline because of preprocessors

chenmusun/easyloggingpp,spthaolt/easyloggingpp,spqr33/easyloggingpp,blankme/easyloggingpp,utiasASRL/easyloggingpp,dreal-deps/easyloggingpp,hellowshinobu/easyloggingpp,utiasASRL/easyloggingpp,hellowshinobu/easyloggingpp,spthaolt/easyloggingpp,dreal-deps/easyloggingpp,blankme/easyloggingpp,blankme/easyloggingpp,spqr33/easyloggingpp,chenmusun/easyloggingpp,arvidsson/easyloggingpp,lisong521/easyloggingpp,lisong521/easyloggingpp,arvidsson/easyloggingpp,hellowshinobu/easyloggingpp,simonhang/easyloggingpp,spthaolt/easyloggingpp,simonhang/easyloggingpp,simonhang/easyloggingpp,spqr33/easyloggingpp,dreal-deps/easyloggingpp,lisong521/easyloggingpp,utiasASRL/easyloggingpp,arvidsson/easyloggingpp,chenmusun/easyloggingpp