Use SplitDateTimeField for date Attribute types

[Dresdn]

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Checklist

• I have double checked that there are no unrelated changes in this pull request (old patches, accidental config files, etc.)
• I have created at least one test case for the changes I have made
• I have updated the documentation for the changes I have made
• I have added my changes to the CHANGELOG.md

Pull Request type

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• Bugfix
• Feature
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no api changes)
• Build related changes
• Documentation content changes
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Related issue(s)

Fixes #261

Other Information

[codecov]

Codecov Report

Merging #321 (c55c9a8) into master (d6a05a0) will decrease coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #321      +/-   ##
==========================================
- Coverage   90.14%   90.12%   -0.02%     
==========================================
  Files          22       22              
  Lines         771      770       -1     
  Branches      138      138              
==========================================
- Hits          695      694       -1     
  Misses         52       52              
  Partials       24       24              

Impacted Files	Coverage Δ
eav/forms.py	78.87% <100.00%> (-0.30%)	⬇️

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