fix(date): fix birthdate

[ST-DDT]

Fixes #2783

• Random CI test failure in date.birthdate for specific age #2783

There are actually two errors:

• One where it is possible to generate the exact same date as the refDate (aka 2020-03-31 and 2000-03-31 for age 20)
• The other lies in the test, where the age is calculated wrongly if the refDate is within a leap year and after Feb 29

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.95%. Comparing base (0fe5af8) to head (600173d).

Additional details and impacted files

@@            Coverage Diff             @@
##             next    #2829      +/-   ##
==========================================
- Coverage   99.95%   99.95%   -0.01%     
==========================================
  Files        2964     2964              
  Lines      212541   212544       +3     
  Branches      600      597       -3     
==========================================
+ Hits       212455   212457       +2     
- Misses         86       87       +1     

Files	Coverage Δ
src/modules/date/index.ts	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes