Fix issue989 concerning timezone specific issue in rest analyses (part 5)

[vincentvanhees]

Fixes #989

Problem was that timestamps were not converted to POSIXct while they should have which caused timezone specific results.

Checklist before merging:

• Existing tests still work (check by running the test suite, e.g. from RStudio).
• Added tests (if you added functionality) or fixed existing test (if you fixed a bug).
• Updated or expanded the documentation.
• Updated release notes in inst/NEWS.Rd with a user-readable summary. Please, include references to relevant issues or PR discussions.
• Added your name to the contributors lists in the DESCRIPTION file, if you think you made a significant contribution.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (7addc38) 78.40% compared to head (dc87d72) 78.41%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #990   +/-   ##
=======================================
  Coverage   78.40%   78.41%           
=======================================
  Files          99       99           
  Lines       13371    13371           
=======================================
+ Hits        10484    10485    +1     
+ Misses       2887     2886    -1     

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