MomentJS and JS Date objects not referring to the same hour

Question

I've got a server instance (NodeJS) that receives a set of objects, and schedules them for sending push notifications to users.

Some of these objects, are periodic, and this periodicity is handled by a string like this:

90=>Mon&Tue&Thu=>16:00

Which is read as:

offset_minutes=>days_of_the_week=>initial_hour

Then, what I do is to check whether the current day matches one of the given days in the string, and then, modify the date to the given hour in the "initial_hour", and finally, substract the "offset_minutes" amount of minutes from the Date object.

Seems straightforward until now, right? Well, not that much. Let's first see the code:

const isToday = weekDays.split("&")
            .map(a => {
                switch (a) {
                    case 'Mon': return 1;
                    case 'Tue': return 2;
                    case 'Wed': return 3;
                    case 'Thu': return 4;
                    case 'Fri': return 5;
                    case 'Sat': return 6;
                    case 'Sun': return 7;
                }
            })
            .some(v => v == currentDay);

        if (isToday) {

            let finalDate = moment(today)
                .set("hour", Number(hour))
                .set("minute", Number(mins));


            if (offset) {
             finalDate.subtract('minutes', Number(offset));
            }


            return finalDate.toDate();

Everything works well, until I do the MomentJS transformations. When I output a Date object with the ".toDate()" method, this object is always set to 2 hours before the expected time . But if I use the.toISOString() method, I get the proper time for all the occurrencies.

I guess that something is wrong with my Date objects, setting them up at a different timezone than the one I have. A couple of examples:

• For the string 90=>Mon&Tue&Thu=>16:00 I get the Date object: 2019-10-14T14:00:11.852Z
• For the string 30=>Mon&Tue&Wed&Thu&Fri&Sat&Sun=>18:30 I get the Date object: 2019-10-14T16:30:11.866Z

I would like to know what's the explanation for such a behavior, and if I can do something to change it so the normal Javascript Date object points to the same hour than my momentjs object, or the.toISOString() output.

Thank you!

Answer 1

The posted code is incomplete and doesn't demonstrate the issue described.

I've reimplemented the code without moment.js as best I can and simplified it. It seems to work fine:

 function parseThing(s) { // Parse input string let b = s.split('=>'); let offset = +b[0]; let days = ['Sun','Mon','Tue','Wed','Thu','Fri','Sat']; let weekDays = b[1].split('&').map(day => days.indexOf(day)); let [hr, min] = b[2].split(':'); // Get a date for today let date = new Date(); // If today included, return an adjusted date if (weekDays.includes(date.getDay())) { date.setHours(hr, min, 0, 0); if (offset) { date.setMinutes(date.getMinutes()+ Number(offset)); } return date; } // If today isn't included, return null return null; } let s0 = '90=>Mon&Tue&Thu=>16:00'; let s1 = '0=>Mon&Tue&Wed&Thu&Fri&Sat&Sun=>18:30'; console.log(parseThing(s0).toString()); console.log(parseThing(s1).toString());

Where the local day is one of those in the string (Mon, Tue, Thu) it returns a Date equivalent to a local time of 17:30, which is 90 minutes offset from 16:00, which seems to be correct.

PS I've changed Sunday to 0 as I can't see any rationale for it to be 7. Also seconds and milliseconds are zeroed too.