Calendar days ago

calendar-days-ago.js

/**
 * Strips the time part away from a date so that it's a pure date (no time). 
 */
function toPureDate(d: Date) : Date {
    return new Date(d.getFullYear(), d.getMonth(), d.getDate(), 0, 0, 0, 0);
}

function hoursAgo(h: number) : Date {
    const now: Date = new Date();
    now.setTime(now.getTime() - h * 60 * 60 * 1000);
    return now;
}

/**
 * The number of calendar days that d2 is behind d1. 
 * If d1 is right now, then 
 *  If d2 = 0 it's today 
 *  If d2 = 1 it's yesterday 
 *  If d2 = 2 it's the day before yesterday 
 *  If d2 = -1 it's tomorrow 
 */
function daysAgo(d1: Date, d2: Date) : number { 
    return Math.round(
        (+toPureDate(d1) - +toPureDate(d2)) / 
        (24 * 60 * 60 * 1000)
    );
}

// Testing. Should "roll over" at a certain point. 
// If it's 8:30pm, then 20 hours ago was 12:30am today (0 days ago)
// but 21 hours ago was 11:30pm yesterday (1 day ago).
// This is all localized to the current time zone.
for (let i = 0; i < 36; i++) {
    const now = new Date();
    const past = hoursAgo(i);
    console.log(i, toPureDate(past), daysAgo(now, past));
}


// Note: I think this solution ignores daylight savings, but eh.
// See https://stackoverflow.com/a/11252167