[BACKEND] Switch `dayjs` usage to utilities to properly use exported `dayjs`

[jji05]

Is your feature request related to a problem? Please describe.

We have a dayjs.ts file where we set up utc, timezone, and our custom parse formatter. This avoids having to duplicate the same measures and so that we can unionise all the date parsing and add tests to ensure functionality is correct.

Describe the solution you'd like

• Avoid using dayjs (default export) at all times and use dayjs.ts file.
• Use date utils whenever possible
• Test all newly created date utils (if necessary)

BEFORE MERGING

• Integration test written for services
• Schemas annotated if adding new models
• Code generation run (hint: pnpm generate:types)
• Appropriate mocks created where possible
• PR Reviewed (For non-trivial changes)
• Changes tested after rebasing on master or merging in master (hint: git fetch origin master:master, then git rebase master or git merge master)
• All required PR checks passing