Verify that index/concat does not change dtype for datetime variables
Verify that index/concat does not change dtype for datetime variables

jcmgray/xarray,cpaulik/xray,jhamman/xarray,chunweiyuan/xarray,rabernat/xray,rabernat/xray,markelg/xray,NicWayand/xray,hetland/xray,xray/xray,kjordahl/xray,pydata/xarray,pydata/xarray,clarkfitzg/xray,petercable/xray,shoyer/xarray,shoyer/xray,markelg/xray,pydata/xarray,cpaulik/xray,jhamman/xarray,drewokane/xray,shoyer/xarray,markelg/xray,clarkfitzg/xray,kjordahl/xray,hetland/xray,jhamman/xarray,jhamman/xray