Set date/time in StageRuntime instead of Preset

[pgeier]

Description

The mars2grib encoder currently sets date and time once in the preset stage. For climate-dt monthly means will produce increasing date/time. To efficiently reuse previously cached grib2 handles, the date/time must be set in the latest stage (runtime).

Contributor Declaration

By opening this pull request, I affirm the following:

• All authors agree to the Contributor License Agreement.
• The code follows the project's coding standards.
• I have performed self-review and added comments where needed.
• I have added or updated tests to verify that my changes are effective and functional.
• I have run all existing tests and confirmed they pass.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 61.95%. Comparing base (f97d405) to head (ed76f11).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #191   +/-   ##
========================================
  Coverage    61.95%   61.95%           
========================================
  Files          303      303           
  Lines        11673    11673           
  Branches      1046     1046           
========================================
  Hits          7232     7232           
  Misses        4441     4441           

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[MircoValentiniECMWF]

There are several ways to achieve this result. The appropriate solution should be decided based on tests involving the other affected projects, as I noted in the Teams chat.