fix time zone bug in headway accuracy logging

[panentheos]

Summary of changes

The headway accuracy logging was using UTC timestamps, which caused it to report the wrong headway values around the peak/off-peak transition.

[github-actions]

Coverage of commit 5b3e1a9

Summary coverage rate:
  lines......: 75.5% (1918 of 2539 lines)
  functions..: 77.0% (519 of 674 functions)
  branches...: no data found

Files changed coverage rate:
                                                             |Lines       |Functions  |Branches    
  Filename                                                   |Rate     Num|Rate    Num|Rate     Num
  =================================================================================================
  lib/headway_analysis/server.ex                             |85.7%     21|83.3%     6|    -      0

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