Start of year and start of fiscal year methods

[BoBoBaSs84]

Is your feature request related to a problem? Please describe.
Quick access via a date value to the start of the year or the start of the fiscal year would be pretty cool. The start of the fiscal year should be definable.

Describe the solution you'd like
Corresponding extension methods would have to be created.

Describe alternatives you've considered
None.

Additional context

public static DateTime StartOfYear(this DateTime dateTime) => new(dateTime.Year, 1, 1);

[BoBoBaSs84]

Consequently, there should also be methods for the end of the year and the end of the fiscal year.