DOC: Thoroughly document datetime issues
DOC: Thoroughly document datetime issues

After understanding the issues with datetime (see gh-568), I have documented my
understanding as comments in the code, so that the next time there are problems
with datetime on some platform, one can quickly figure out what is going on
from the code + comments.

WarrenWeckesser/numpy,moreati/numpy,Anwesh43/numpy,larsmans/numpy,behzadnouri/numpy,dch312/numpy,tacaswell/numpy,moreati/numpy,tacaswell/numpy,skymanaditya1/numpy,behzadnouri/numpy,jorisvandenbossche/numpy,ESSS/numpy,simongibbons/numpy,immerrr/numpy,musically-ut/numpy,ChanderG/numpy,endolith/numpy,abalkin/numpy,naritta/numpy,rhythmsosad/numpy,pizzathief/numpy,joferkington/numpy,Srisai85/numpy,immerrr/numpy,skwbc/numpy,mhvk/numpy,felipebetancur/numpy,sinhrks/numpy,andsor/numpy,kirillzhuravlev/numpy,charris/numpy,shoyer/numpy,mathdd/numpy,musically-ut/numpy,ewmoore/numpy,jankoslavic/numpy,kiwifb/numpy,has2k1/numpy,anntzer/numpy,leifdenby/numpy,mingwpy/numpy,chatcannon/numpy,pbrod/numpy,kiwifb/numpy,pyparallel/numpy,kirillzhuravlev/numpy,mingwpy/numpy,mattip/numpy,cowlicks/numpy,MichaelAquilina/numpy,pbrod/numpy,ewmoore/numpy,ContinuumIO/numpy,WillieMaddox/numpy,immerrr/numpy,ESSS/numpy,abalkin/numpy,Anwesh43/numpy,rherault-insa/numpy,numpy/numpy,rherault-insa/numpy,ViralLeadership/numpy,bmorris3/numpy,dwillmer/numpy,ChanderG/numpy,ajdawson/numpy,SiccarPoint/numpy,rgommers/numpy,NextThought/pypy-numpy,rhythmsosad/numpy,GaZ3ll3/numpy,pbrod/numpy,rgommers/numpy,dch312/numpy,trankmichael/numpy,mindw/numpy,moreati/numpy,Eric89GXL/numpy,has2k1/numpy,Anwesh43/numpy,Srisai85/numpy,Eric89GXL/numpy,njase/numpy,leifdenby/numpy,simongibbons/numpy,naritta/numpy,ahaldane/numpy,MSeifert04/numpy,musically-ut/numpy,MSeifert04/numpy,nguyentu1602/numpy,SunghanKim/numpy,sinhrks/numpy,mortada/numpy,rherault-insa/numpy,NextThought/pypy-numpy,ContinuumIO/numpy,larsmans/numpy,rmcgibbo/numpy,dimasad/numpy,tdsmith/numpy,nguyentu1602/numpy,mingwpy/numpy,has2k1/numpy,mindw/numpy,seberg/numpy,anntzer/numpy,ahaldane/numpy,andsor/numpy,jschueller/numpy,pyparallel/numpy,Yusa95/numpy,endolith/numpy,bmorris3/numpy,mwiebe/numpy,brandon-rhodes/numpy,drasmuss/numpy,MSeifert04/numpy,dimasad/numpy,rudimeier/numpy,CMartelLML/numpy,KaelChen/numpy,ewmoore/numpy,nguyentu1602/numpy,argriffing/numpy,joferkington/numpy,jakirkham/numpy,kiwifb/numpy,ChanderG/numpy,pizzathief/numpy,ChristopherHogan/numpy,rhythmsosad/numpy,mindw/numpy,sigma-random/numpy,shoyer/numpy,githubmlai/numpy,KaelChen/numpy,naritta/numpy,Dapid/numpy,sonnyhu/numpy,ContinuumIO/numpy,njase/numpy,mortada/numpy,ViralLeadership/numpy,tdsmith/numpy,hainm/numpy,nguyentu1602/numpy,argriffing/numpy,empeeu/numpy,stuarteberg/numpy,jonathanunderwood/numpy,hainm/numpy,naritta/numpy,leifdenby/numpy,SunghanKim/numpy,MaPePeR/numpy,rajathkumarmp/numpy,shoyer/numpy,grlee77/numpy,b-carter/numpy,sigma-random/numpy,grlee77/numpy,rgommers/numpy,dwillmer/numpy,dato-code/numpy,grlee77/numpy,mattip/numpy,WillieMaddox/numpy,kirillzhuravlev/numpy,stuarteberg/numpy,dch312/numpy,argriffing/numpy,ajdawson/numpy,jorisvandenbossche/numpy,joferkington/numpy,ekalosak/numpy,MichaelAquilina/numpy,WarrenWeckesser/numpy,groutr/numpy,skwbc/numpy,KaelChen/numpy,chiffa/numpy,ogrisel/numpy,KaelChen/numpy,ahaldane/numpy,mhvk/numpy,chatcannon/numpy,nbeaver/numpy,stuarteberg/numpy,mattip/numpy,jakirkham/numpy,solarjoe/numpy,chiffa/numpy,githubmlai/numpy,tynn/numpy,mindw/numpy,njase/numpy,pyparallel/numpy,ChristopherHogan/numpy,maniteja123/numpy,MaPePeR/numpy,joferkington/numpy,mhvk/numpy,Anwesh43/numpy,madphysicist/numpy,seberg/numpy,Eric89GXL/numpy,mhvk/numpy,gmcastil/numpy,dwillmer/numpy,grlee77/numpy,solarjoe/numpy,pbrod/numpy,SiccarPoint/numpy,Linkid/numpy,larsmans/numpy,brandon-rhodes/numpy,ewmoore/numpy,rudimeier/numpy,madphysicist/numpy,pdebuyl/numpy,brandon-rhodes/numpy,seberg/numpy,has2k1/numpy,pdebuyl/numpy,BabeNovelty/numpy,ekalosak/numpy,skymanaditya1/numpy,drasmuss/numpy,ahaldane/numpy,AustereCuriosity/numpy,jschueller/numpy,Dapid/numpy,dato-code/numpy,chiffa/numpy,skymanaditya1/numpy,sinhrks/numpy,mathdd/numpy,embray/numpy,githubmlai/numpy,simongibbons/numpy,ekalosak/numpy,shoyer/numpy,jorisvandenbossche/numpy,utke1/numpy,tacaswell/numpy,abalkin/numpy,nbeaver/numpy,trankmichael/numpy,mortada/numpy,shoyer/numpy,tynn/numpy,BabeNovelty/numpy,larsmans/numpy,charris/numpy,ssanderson/numpy,embray/numpy,gmcastil/numpy,rajathkumarmp/numpy,MSeifert04/numpy,jonathanunderwood/numpy,yiakwy/numpy,musically-ut/numpy,sigma-random/numpy,ESSS/numpy,SunghanKim/numpy,nbeaver/numpy,madphysicist/numpy,rhythmsosad/numpy,groutr/numpy,BMJHayward/numpy,ogrisel/numpy,BMJHayward/numpy,charris/numpy,dato-code/numpy,bmorris3/numpy,numpy/numpy,ewmoore/numpy,trankmichael/numpy,groutr/numpy,Dapid/numpy,drasmuss/numpy,felipebetancur/numpy,rgommers/numpy,MichaelAquilina/numpy,Linkid/numpy,cowlicks/numpy,mortada/numpy,sonnyhu/numpy,felipebetancur/numpy,tdsmith/numpy,utke1/numpy,brandon-rhodes/numpy,empeeu/numpy,yiakwy/numpy,Eric89GXL/numpy,bringingheavendown/numpy,skwbc/numpy,githubmlai/numpy,cjermain/numpy,gfyoung/numpy,mwiebe/numpy,maniteja123/numpy,embray/numpy,yiakwy/numpy,tdsmith/numpy,jankoslavic/numpy,kirillzhuravlev/numpy,utke1/numpy,rmcgibbo/numpy,endolith/numpy,BMJHayward/numpy,ChristopherHogan/numpy,anntzer/numpy,jankoslavic/numpy,ssanderson/numpy,MSeifert04/numpy,cjermain/numpy,GrimDerp/numpy,dato-code/numpy,sigma-random/numpy,pdebuyl/numpy,BabeNovelty/numpy,jakirkham/numpy,jankoslavic/numpy,WarrenWeckesser/numpy,skymanaditya1/numpy,CMartelLML/numpy,yiakwy/numpy,maniteja123/numpy,BabeNovelty/numpy,endolith/numpy,mwiebe/numpy,WillieMaddox/numpy,mattip/numpy,pizzathief/numpy,ChristopherHogan/numpy,felipebetancur/numpy,sonnyhu/numpy,Srisai85/numpy,andsor/numpy,sonnyhu/numpy,rajathkumarmp/numpy,solarjoe/numpy,seberg/numpy,cowlicks/numpy,grlee77/numpy,ddasilva/numpy,bertrand-l/numpy,AustereCuriosity/numpy,madphysicist/numpy,dimasad/numpy,jonathanunderwood/numpy,b-carter/numpy,ajdawson/numpy,ogrisel/numpy,jakirkham/numpy,jschueller/numpy,Linkid/numpy,Yusa95/numpy,GaZ3ll3/numpy,bringingheavendown/numpy,Yusa95/numpy,charris/numpy,sinhrks/numpy,pizzathief/numpy,CMartelLML/numpy,cjermain/numpy,ahaldane/numpy,rudimeier/numpy,gfyoung/numpy,rmcgibbo/numpy,jorisvandenbossche/numpy,tynn/numpy,NextThought/pypy-numpy,cjermain/numpy,ssanderson/numpy,ChanderG/numpy,SunghanKim/numpy,stuarteberg/numpy,embray/numpy,bertrand-l/numpy,dwillmer/numpy,pbrod/numpy,NextThought/pypy-numpy,rmcgibbo/numpy,pdebuyl/numpy,trankmichael/numpy,MichaelAquilina/numpy,bringingheavendown/numpy,numpy/numpy,andsor/numpy,mathdd/numpy,hainm/numpy,rudimeier/numpy,GrimDerp/numpy,dimasad/numpy,ajdawson/numpy,immerrr/numpy,Yusa95/numpy,SiccarPoint/numpy,BMJHayward/numpy,chatcannon/numpy,empeeu/numpy,WarrenWeckesser/numpy,mingwpy/numpy,GaZ3ll3/numpy,AustereCuriosity/numpy,GrimDerp/numpy,rajathkumarmp/numpy,gmcastil/numpy,jakirkham/numpy,SiccarPoint/numpy,jorisvandenbossche/numpy,pizzathief/numpy,simongibbons/numpy,ddasilva/numpy,WarrenWeckesser/numpy,ekalosak/numpy,empeeu/numpy,dch312/numpy,cowlicks/numpy,Linkid/numpy,hainm/numpy,mhvk/numpy,madphysicist/numpy,MaPePeR/numpy,CMartelLML/numpy,Srisai85/numpy,embray/numpy,behzadnouri/numpy,GrimDerp/numpy,numpy/numpy,ogrisel/numpy,GaZ3ll3/numpy,MaPePeR/numpy,bmorris3/numpy,jschueller/numpy,ogrisel/numpy,b-carter/numpy,simongibbons/numpy,anntzer/numpy,ddasilva/numpy,bertrand-l/numpy,ViralLeadership/numpy,mathdd/numpy,gfyoung/numpy