Make testscript date_bar.jl deterministic wrt date run

[non-Jedi]

Non-deterministic testscript discovered while regression testing #1276.

Contributor checklist:

• I've updated the documentation to reflect these changes
• I've added an entry to NEWS.md
• I've added and/or updated the unit tests
• I've run the regression tests
• I've squash'ed or fixup'ed junk commits with git-rebase
• I've built the docs and confirmed these changes don't cause new errors

[codecov-io]

Codecov Report

Merging #1279 into master will decrease coverage by 0.25%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1279      +/-   ##
==========================================
- Coverage   90.33%   90.08%   -0.26%     
==========================================
  Files          36       36              
  Lines        3943     3954      +11     
==========================================
  Hits         3562     3562              
- Misses        381      392      +11

Impacted Files	Coverage Δ
src/aesthetics.jl	80.17% <0%> (-2.13%)	⬇️
src/scale.jl	97.41% <0%> (-1.28%)	⬇️
src/theme.jl	71.18% <0%> (-1.23%)	⬇️
src/mapping.jl	84.61% <0%> (-0.83%)	⬇️
src/misc.jl	65.77% <0%> (-0.45%)	⬇️
src/Gadfly.jl	78.58% <0%> (-0.4%)	⬇️

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[bjarthur]

thanks!