fix: remove formatDatetime

[keita-determined]

Ticket

CM-550

Description

Remove formatDatetime because it would cause inconsistent date/datetime format in the future, and no need to manually manipulate dateString.

Test Plan

• Check if all date strings are formatted the same as before
  • Model header
  • Model version header
  • Timestamp in log viewer
  • Checkpoint modal

Checklist

• Changes have been manually QA'd
• New features have been approved by the corresponding PM
• User-facing API changes have the "User-facing API Change" label
• Release notes have been added as a separate file under docs/release-notes/
  See Release Note for details.
• Licenses have been included for new code which was copied and/or modified from any external code

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[codecov]

Codecov Report

Attention: Patch coverage is 54.54545% with 5 lines in your changes missing coverage. Please review.

Project coverage is 50.53%. Comparing base (987b2a5) to head (95a4bac).
Report is 7 commits behind head on main.

Files with missing lines	Patch %	Lines
webui/react/src/pages/ModelDetails/ModelHeader.tsx	0.00%	1 Missing and 1 partial ⚠️
...c/pages/ModelVersionDetails/ModelVersionHeader.tsx	0.00%	1 Missing and 1 partial ⚠️
webui/react/src/components/TrialLogPreview.tsx	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #10011      +/-   ##
==========================================
- Coverage   54.57%   50.53%   -4.05%     
==========================================
  Files        1259      949     -310     
  Lines      157233   128576   -28657     
  Branches     3620     3616       -4     
==========================================
- Hits        85807    64971   -20836     
+ Misses      71293    63472    -7821     
  Partials      133      133              

Flag	Coverage Δ
harness	?
web	54.32% <54.54%> (-0.02%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
webui/react/src/components/CheckpointModal.tsx	80.61% <100.00%> (+0.34%)	⬆️
webui/react/src/utils/datetime.ts	68.81% <ø> (-7.22%)	⬇️
webui/react/src/components/TrialLogPreview.tsx	71.32% <0.00%> (-0.21%)	⬇️
webui/react/src/pages/ModelDetails/ModelHeader.tsx	0.00% <0.00%> (ø)
...c/pages/ModelVersionDetails/ModelVersionHeader.tsx	0.00% <0.00%> (ø)

... and 310 files with indirect coverage changes

[EmilyBonar]

LGTM, though I'm not 100% clear on the reasoning behind the removal.

[keita-determined]

LGTM, though I'm not 100% clear on the reasoning behind the removal.

1. To avoid the manual calculation. For example, we can do the same thing without having stripTimezone and formatDatetime with dayjs, so these are unnecessary wrappers. using dayjs is more reliable. Especially, datetime handling is complicated. it's safer to use dayjs instead.
2. so far, outputUTC are not used, but it might cause timezone inconsistency in the web app. and even if we wanna do this, we can directly use dayjs without these functions.