Refactor date and time methods into utility file

[FergusMok]

The frontend uses DateFormat/TimeFormat, Moment and epoch timestamps, and even Date. However, functions to parse and manipulate between the data types are scattered in different files and folders.

Examples that can benefit from refactoring:

• Put methods to convert between the types into one single utility file E.g getDateTimeAtTimezone (conversion from timestamp to date/time format) implemented only in Session Edit page
• Time-related magic numbers being declared locally repeatedly, or even just used plainly as numbers throughout the repo

[tau-bar]

Hello, @FergusMok is it okay if I try working on this?

[FergusMok]

@tau-bar Yes please go ahead :)

[tau-bar]

@FergusMok Could you help me in reviewing the PR?