Remove hardcoded time zones from code

We have hardcoded time zones in the code base (both client and server). These should be removed.

**Purpose**
To make the system more robust against time zone changes, specifically, the BC change to permanently set BC Pacific Time to -7.

**Acceptance Criteria**
- [ ] Remove all hardcoded time zones from code, e.g., "Canada/Pacific"
- [ ] Make sure code is updated so that all dates that had hardcoded time zones function as they did before

** Additional context**
- These are generally set using the Moment.js or dayjs `tz()` function
- Resulted from #2944

Confirm this task meets our team [**Definition of Ready (DoR)**](https://www.figma.com/board/XmNkN600aAVWFCBmSDdt10/Team-Documents?node-id=1-2&t=Xjd6nHZZ7YlnoOrD-4) before moving to the **Next Sprint** column.

Confirm this task meets our team [**Definition of Done (DoD)**](https://www.figma.com/board/XmNkN600aAVWFCBmSDdt10/Team-Documents?node-id=3-21&t=Xjd6nHZZ7YlnoOrD-4) before moving to the **Done** column.

Search both the client and server code for Moment.js or dayjs tz() calls and hardcoded values such as "Canada/Pacific". Review each date-handling location and verify that dates retain their previous behavior without hardcoded time zones. Done means all hardcoded time zones are removed and the affected date behavior remains unchanged.

Written by the indexing model from the issue text.