use chrono for day of week labels

move `window::get_sunday` to `time::get_calender_first`

cosmic-applet-time/src/time.rs

@@ -0,0 +1,8 @@
+use chrono::{Datelike, Days, NaiveDate, Weekday};
+
+/// Gets the first date that will be visible on the calender
+pub fn get_calender_first(year: i32, month: u32, from_weekday: Weekday) -> NaiveDate {
+    let date = NaiveDate::from_ymd_opt(year, month, 1).unwrap();
+    let num_days = (date.weekday() as u32 + 7 - from_weekday as u32) % 7; // chrono::Weekday.num_days_from
+    date.checked_sub_days(Days::new(num_days as u64)).unwrap()
+}