Reporting: Transactions List - Simplify how time zones are calculated and passed on

[nagpai]

Describe the bug

( This is a WIP draft. Details and links are yet to be plugged in ) The transaction cache on the server saves time stamps in UTC.

The transactions list in reports ( Payment > Transactions ) and all other reports that use it ( e.g. Payment activity widget ), shows the time in the website time zone.

The filter queries, that are in the same website time zone, are eventually converted to UTC while querying the transaction cache. We however see this is done in a complex way where there is also user's local browser time zone involved.

We need to explore if getting the user's browser time zone is indeed required, or if there is a scope to simplify this further.

Additional context

• The topic came up while resolving a bug related to the Payment activity widget
• The PR above includes adjustment to how we adjust for the website time zone for the Payment widget numbers, and the date params to transaction list that appears when we click the view report link on the widget.

[haszari]

@nagpai this issue is quite vague and hard to action. How can we make it clearer and more actionable?

Here's my understanding:

• List_Transactions class in plugin adjusts date filter values to user's timezone:

https://github.com/Automattic/woocommerce-payments/blob/bebe14e87eb2a4604c82dd212221efc1854302c1/includes/core/server/request/class-list-transactions.php#L80-L81

• We don't know why it does this, or what purpose it serves. Other similar classes don't convert timezone. For example, List_Deposits, List_Disputes, and List_Documents all don't convert timezone:

https://github.com/Automattic/woocommerce-payments/blob/bebe14e87eb2a4604c82dd212221efc1854302c1/includes/core/server/request/class-list-deposits.php#L48-L50

https://github.com/Automattic/woocommerce-payments/blob/bebe14e87eb2a4604c82dd212221efc1854302c1/includes/core/server/request/class-list-disputes.php#L54-L56

https://github.com/Automattic/woocommerce-payments/blob/bebe14e87eb2a4604c82dd212221efc1854302c1/includes/core/server/request/class-list-documents.php#L46-L48

• The goal of this issue is to investigate this and find out why transactions are handled inconsistently, and if this should applied to other similar requests, or if List_Transactions should pass time values through as-is.
  • Are there clues in git history for when this was added – i.e. to fix a bug or because of a design decision?
  • Is there code elsewhere that applies timezone corrections inconsistently, that has led to inconsistency in the PHP request classes?
  • If we remove the timezone adjustment in List_Transactions, what UI or behaviour is affected, is it "broken"?
• And once that research spike is concluded, make any fixes/changes to code (or log issues / projects to do so).