'Dates' module errors

[hustf]

On Windows, you can create a date like this:

$ julia7
               _
   _       _ _(_)_     |  A fresh approach to technical computing
  (_)     | (_) (_)    |  Documentation: https://docs.julialang.org
   _ _   _| |_  __ _   |  Type "?help" for help.
  | | | | | | |/ _` |  |
  | | |_| | | | (_| |  |  Version 0.7.0-DEV.2549 (2017-11-20 18:47 UTC)
 _/ |\__'_|_|_|\__'_|  |  Commit 0f906bacff* (2 days old master)
|__/                   |  x86_64-w64-mingw32

julia> Dates.format(now(Dates.UTC), Dates.RFC1123Format)
"Thu, 23 Nov 2017 18:00:07"

...whereas on OSX (downloaded binary):

 _/ |\__'_|_|_|\__'_|  |  Commit dd0bf99db7 (0 days old master)
|__/                   |  x86_64-apple-darwin14.5.0

julia> Dates.format(now(Dates.UTC), Dates.RFC1123Format)
WARNING: Base.Dates is deprecated, run `using Dates` instead.
  likely near no file:0
ERROR: type Void has no field format

Travis has the OSX behaviour on both OSX and linux.

Info on the windows build:
Julia Version 0.7.0-DEV.2549
Commit 0f906bacff* (2017-11-20 18:47 UTC)
Platform Info:
  OS: Windows (x86_64-w64-mingw32)
      Microsoft Windows [Version 10.0.16299.64]
  CPU: Intel(R) Core(TM) i7-7700HQ CPU @ 2.80GHz:
              speed         user       nice        sys       idle        irq ticks
       #1  2808 MHz    7336906          0    7923093  154653281    3005546 ticks
       #2  2808 MHz    4263531          0    1632250  164017359      69343 ticks
       #3  2808 MHz    7215296          0    2875781  159822062      73015 ticks
       #4  2808 MHz    4428640          0    1217531  164266968      27796 ticks
       #5  2808 MHz    6581171          0    2697093  160634875      58937 ticks
       #6  2808 MHz    5434984          0    1500640  162977515      38578 ticks
       #7  2808 MHz    7654687          0    2207500  160050953      58390 ticks
       #8  2808 MHz    5734906          0    1761656  162416562      41437 ticks

  Memory: 7.886325836181641 GB (3955.953125 MB free)
  Uptime: 590001.6943746 sec
  Load Avg:  0.0  0.0  0.0
  WORD_SIZE: 64
  BLAS: libopenblas (USE64BITINT DYNAMIC_ARCH NO_AFFINITY Haswell)
  LAPACK: libopenblas64_
  LIBM: libopenlibm
  LLVM: libLLVM-3.9.1 (ORCJIT, skylake)

[fredrikekre]

The printed warning tells you what to do; using Dates. See #24459