Function days_from_civil 

pub fn days_from_civil(year: i64, month: u32, day: u32) -> i64

Days since 1970-01-01 for a proleptic-Gregorian year/month/day (month 1..=12, day 1..=31). Day-of-month overflow rolls forward (Feb 30 is Mar 2 in a common year), which callers clamping with days_in_month never produce.

use tilo_ui::date::days_from_civil;
assert_eq!(days_from_civil(1970, 1, 1), 0);
assert_eq!(days_from_civil(2000, 1, 1), 10_957);
assert_eq!(days_from_civil(1969, 12, 31), -1);