Some date algorithms

[lpil]

https://howardhinnant.github.io/date_algorithms.html

Unclear if any of these will be useful here, but I wanted to make note of them. It may be that these are more suited to community package.

[dzuk-mutant]

I would personally find these useful (eg. the other day I was thinking of using this package, but I noticed I couldn't derive things like days of the week from a Timestamp so I tried gtempo instead) , but if they don't seem good for the core package I might try to make something myself at some point.

[lpil]

The expectation is that community packages provide further functionality for the time types provided. It may be that some functionality is promoted into this library, but supplementary libraries are the way to go for the immediate future.

[NNBnh]

I made a quick implement of most algorithms in the link to gleam on my phone ^^:

pub type WeekDay {
  Monday
  Tuesday
  Wednesday
  Thursday
  Friday
  Saturday
  Sunday
}

pub fn days_of_month(month month: Month, year year: Int) -> Int {
  case month {
	January -> 31
	February ->
	  case is_leap_year(year) {
        True -> 29
        False -> 28
      }
	March -> 31
	April -> 30
	May -> 31
	June -> 30
	July -> 31
	August -> 31
	September -> 30
	October -> 31
    November -> 30
    December -> 31
  }
}

pub fn date_to_weekday(date: Date) -> WeekDay {
  let #(seconds, _nanoseconds) =
    date
    |> timestamp.from_calendar(TimeOfDay(0, 0, 0, 0), utc_offset)
    |> timestamp.to_unix_seconds_and_nanoseconds()

  let days = seconds / 86_400
  
  let weekday = case days >= 4 {
    True -> { days + 4 } % 7
    False -> { days + 5 } % 7 + 6
  }
  
  case weekday {
    0 -> Sunday
    1 -> Monday
    2 -> Tuesday
    3 -> Wednesday
    4 -> Thursday
    5 -> Friday
    6 -> Saturday
    _ -> panic
  }
}

pub fn weekday_to_string(weekday: WeekDay) -> String {
  case weekday {
    Monday -> "Monday"
    Tuesday -> "Tuesday"
    Wednesday -> "Wednesday"
    Thursday -> "Thursday"
    Friday -> "Friday"
    Saturday -> "Saturday"
    Sunday -> "Sunday"
  }
}

I surprise that WeekDay type is not include in the package when it's so fundamental and universal to handle date.

I'm willing to make a helper packages for this but not sure if I should add WeekDay to my package or wait for the official package to do so, then use the WeekDay from this package to make my functions input/output types easier to integrate and more widely accepted/adapted.

[lpil]

Number to weekday is tricky as the mapping is regional. For example, the one you've used is what USA uses, but not in Europe.

If we did add it I think we'd likely use ISO 8601, which has Monday as the first day.