Continuous time updates

Add TODO.md in src/components/datetime. This file give detailed information about a refactoring of the DateTimeController that would be nice to do in the future.

src/components/datetime/TODO.md

@@ -0,0 +1,41 @@
+# Refactoring needed
+
+## Context
+
+The [PR #2041 - Continuous time updates](https://github.com/InfiniTimeOrg/InfiniTime/pull/2041) highlighted some
+limitations in the design of DateTimeController: the granularity of the time returned by `DateTime::CurrentDateTime()`
+is limited by the frequency at which SystemTask calls `DateTime::UpdateTime()`, which is currently set to 100ms.
+
+@mark9064 provided more details
+in [this comment](https://github.com/InfiniTimeOrg/InfiniTime/pull/2041#issuecomment-2048528967).
+
+The [PR #2041 - Continuous time updates](https://github.com/InfiniTimeOrg/InfiniTime/pull/2041) provided some changes
+to `DateTime` controller that improves the granularity of the time returned by `DateTime::CurrentDateTime()`.
+
+However, the review showed that `DateTime` cannot be `const` anymore, even when it's only used to get the current time,
+since `DateTime::CurrentDateTime()` changes the internal state of the instance.
+
+We tried to identify alternative implementation that would have maintained the "const correctness" but we eventually
+figured that this would lead to a re-design of `DateTime` which was out of scope of the initial PR (Continuous time
+updates and always on display).
+
+So we decided to merge this PR #2041 and agree to fix/improve `DateTime` later on.
+
+## What needs to be done?
+
+Improve/redesign `DateTime` so that it
+
+* provides a very granular (ideally down to the millisecond) date and time via `CurrentDateTime()`.
+* can be declared/passed as `const` when it's only used to **get** the time.
+* limits the use of mutex as much as possible (an ideal implementation would not use any mutex, but this might not be
+  possible).
+* improves the design of `DateTime::Seconds()`, `DateTime::Minutes()`, `DateTime::Hours()`, etc as
+  explained [in this comment](https://github.com/InfiniTimeOrg/InfiniTime/pull/2054#pullrequestreview-2037033105).
+
+Once this redesign is implemented, all instances/references to `DateTime` should be reviewed and updated to use `const`
+where appropriate.
+
+Please check the following PR to get more context about this redesign:
+
+* [#2041 - Continuous time updates by @mark9064](https://github.com/InfiniTimeOrg/InfiniTime/pull/2041)
+* [#2054 - Continuous time update - Alternative implementation to #2041 by @JF002](https://github.com/InfiniTimeOrg/InfiniTime/pull/2054)