gistfile1.jl

julia> reports["Lat"]
1130-element DataArray{Float64,1}:
 40.2717
 40.2661
 40.2925
 40.2498
 40.2765
 40.2704
 40.2644
 40.2731
 40.274 
 40.2691
  ⋮     
 40.2619
 40.2648
 40.2831
 40.2632
 40.2577
 40.2624
 40.2762
 40.2786
 40.2541
 40.2718

## Above the column is a DataArray{Float64,1}

julia> reports["End"]
1130-element DataArray{Any,1}:
 2014-01-02T19:31:00 UTC
 2014-01-02T19:16:00 UTC
 2014-01-02T11:06:00 UTC
 2014-01-02T11:00:00 UTC
 2014-01-02T07:55:00 UTC
 2014-01-02T02:02:00 UTC
 2014-01-02T01:10:00 UTC
 2014-01-01T23:23:00 UTC
 2014-01-01T21:00:00 UTC
 2014-01-01T15:47:00 UTC
 ⋮                      
 2013-11-01T14:25:00 UTC
 2013-11-01T12:40:00 UTC
 2013-11-01T12:14:00 UTC
 2013-11-01T10:43:00 UTC
 2013-11-01T10:13:00 UTC
 2013-11-01T10:00:00 UTC
 2013-11-01T07:45:00 UTC
 2013-11-01T07:37:00 UTC
 2013-11-01T04:30:00 UTC
 2013-11-01T00:44:00 UTC

## Everything in this column is a DateTime, but it's a DataArray{Any,1}
##
## How would I change that?

With the latest (not released) DataArrays code, you could do:

reports["End"] = convert(DataVector{DateTime}, reports["End"])

This currently isn't fully supported because of a subtle bug in Julia.