[frontend] fix simulation start time validation (#1360)

[guillaumejparis]

Close #1360

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 28.86%. Comparing base (c5d7361) to head (c2f9d79).
Report is 1 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #1362      +/-   ##
============================================
+ Coverage     28.63%   28.86%   +0.23%     
- Complexity     1319     1323       +4     
============================================
  Files           526      526              
  Lines         13726    13726              
  Branches        814      814              
============================================
+ Hits           3930     3962      +32     
+ Misses         9611     9579      -32     
  Partials        185      185              

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