fix: switch to valueof when doing unix conversion

[brobro10000]

When doing comparisons of date using dates and timestamp, use the milliseconds conversion to increase precision. With this change the start date logic comparison of "Is the earliest of 2 start dates (enrollStart OR start) before the subsidy expiration date minus 14 day threshold.", the resolved logic is now correct.

For all changes

• Ensure adequate tests are in place (or reviewed existing tests cover changes)

Only if submitting a visual change

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[codecov]

Codecov Report

Attention: Patch coverage is 84.00000% with 4 lines in your changes missing coverage. Please review.

Project coverage is 85.59%. Comparing base (e7aaa47) to head (315753d).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
...components/learner-credit-management/data/utils.js	84.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1320      +/-   ##
==========================================
- Coverage   85.60%   85.59%   -0.01%     
==========================================
  Files         575      575              
  Lines       12672    12678       +6     
  Branches     2691     2646      -45     
==========================================
+ Hits        10848    10852       +4     
- Misses       1762     1766       +4     
+ Partials       62       60       -2     

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[adamstankiewicz]

LGTM, with a suggestion to update a comment related to late enrollment.