[stable5.1] refactor: don't rely on locale when parsing date

[backportbot]

Backport of PR #6736

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 4 lines in your changes missing coverage. Please review.

Project coverage is 14.54%. Comparing base (2546905) to head (e7a1e6a).
Report is 8 commits behind head on stable5.1.

Files with missing lines	Patch %	Lines
src/mixins/EditorMixin.js	0.00%	4 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##           stable5.1    #6738      +/-   ##
=============================================
- Coverage      14.55%   14.54%   -0.01%     
=============================================
  Files            210      210              
  Lines           9814     9818       +4     
  Branches        2308     2309       +1     
=============================================
  Hits            1428     1428              
- Misses          8060     8064       +4     
  Partials         326      326              

Flag	Coverage Δ
javascript	14.54% <0.00%> (-0.01%)	⬇️

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