Fix date and date-time format

[francescopersico]

Fix string format as https://swagger.io/docs/specification/data-models/data-types/#string

[codecov-commenter]

Codecov Report

Merging #184 into master will decrease coverage by 0.21%.
The diff coverage is 88.88%.

@@            Coverage Diff             @@
##           master     #184      +/-   ##
==========================================
- Coverage   46.85%   46.64%   -0.22%     
==========================================
  Files           1        1              
  Lines         254      253       -1     
  Branches       81       82       +1     
==========================================
- Hits          119      118       -1     
  Misses        133      133              
  Partials        2        2              

Impacted Files	Coverage Δ
src/gen-api-models.ts	46.64% <88.88%> (-0.22%)	⬇️

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