Use Dates.format in `yearmon`

[asinghvi17]

[codecov-commenter]

Codecov Report

Merging #76 (c286764) into main (f0ab22b) will decrease coverage by 0.04%. The diff coverage is 100.00%.

:exclamation: Current head c286764 differs from pull request most recent head 349ff30. Consider uploading reports for the commit 349ff30 to get more accurate results

@@            Coverage Diff             @@
##             main      #76      +/-   ##
==========================================
- Coverage   96.96%   96.93%   -0.04%     
==========================================
  Files          16       16              
  Lines         363      359       -4     
==========================================
- Hits          352      348       -4     
  Misses         11       11              

Impacted Files	Coverage Δ
src/other/date_to_int.jl	100.00% <100.00%> (ø)

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