Add date- and time-related utilities and extensions

[ric15ni]

Proposed changes

Add a series of date- and time-related utilities and extensions:

• constants for several "magic numbers" (ticks per hour, ticks per day, milliseconds per day, etc.);
• utility methods to get the first and last day in a week or month, either relative to the current date, or as extension methods on both DateTime and DateOnly;
• extension methods for DateTime to get the last day in a week or month, at the latest possible time (last tick), useful to define inclusive ranges (for example as parameters to a SQL query using a BETWEEN clause).

Checklist of related issues / discussions

There was no time for opening an issue, let alone discussing it. The added code comes from production projects, where it has been copypasted more times than I'll ever admit. 😬

Types of changes

This pull request introduces the following types of changes:

• Bug fix
• New feature
• Test addition / update (no changes to non-test code)
• Refactor (no changes in public API syntax or semantics)
• Performance improvement (no changes in public API syntax or semantics)
• Documentation (docs directory) update
• Dependency addition / update
• Changes to the build scripts
• Changes to CI (workflows, bot / app configurations)
• Changes to repository files (.gitattributes, .gitignore)
• Other

Breaking changes

This pull request introduces breaking changes:

• Yes
• No

Checklist

• For all types of changes:
  • I have read the Code of Conduct and agree to be bound by it
  • I have read the Contributor Guide and agree to follow it
• For code changes only:
  • The project builds on my machine, via the provided build script, with zero warnings
  • I have added tests that prove my feature works / my fix is effective
  • I have added / modified XML documentation according to changes in code
  • I have checked that all the links I added or modified in XML documentation point to their intended destination
• For documentation changes (docs directory) only:
  • I have built and tested documentation locally
  • I have checked that all the links I added or modified point to their intended destination

[rdeago]

LGTM 👍