feat(dates): add basic dates logic

[bestdan]

No description provided.

[codecov]

Codecov Report

Merging #17 (e896df6) into main (7a2d416) will decrease coverage by 5.15%.
The diff coverage is 84.37%.

❗ Current head e896df6 differs from pull request most recent head e712564. Consider uploading reports for the commit e712564 to get more accurate results

@@            Coverage Diff             @@
##             main      #17      +/-   ##
==========================================
- Coverage   96.92%   91.76%   -5.16%     
==========================================
  Files           3        5       +2     
  Lines          65       85      +20     
==========================================
+ Hits           63       78      +15     
- Misses          2        7       +5     

Impacted Files	Coverage Δ
lib/src/base/return_period.dart	50.00% <50.00%> (ø)
lib/src/base/return.dart	90.00% <71.42%> (-10.00%)	⬇️
lib/src/base/calc_trading_period.dart	100.00% <100.00%> (ø)
lib/src/base/return_stream.dart	100.00% <100.00%> (ø)

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