I think you can use [moment query functions]:https://momentjs.com/docs/#/query/

Take into account that moment works with specific param formats, so you cannot give it standalone objects.

Considered that, you'll need the diff function and parse apropiately remindTime.date and remindTime.time:

To get the difference in milliseconds, use moment#diff like you would use moment#from.

var a = moment([2007, 0, 29]);
var b = moment([2007, 0, 28]);
a.diff(b) // 86400000

And to get the difference in another unit of measurement, pass that measurement as the second argument.

var a = moment([2007, 0, 29]);
var b = moment([2007, 0, 28]);
a.diff(b, 'days') // 1

An example for this could be:

//given
const remindTime1 =
        {
            _id: new ObjectId("614fa5ed18b365aa83099183"),
            title: 'Third Message',
            description: 'Item 3',
            remindTime: { date: '2021-09-26', time: '7:43:21 pm' },
            isComplete: false,
        }
// and also given another object for comparing times and dates

const remindTime2 =
        {
            _id: new ObjectId("614fa5ed18b365aa83099183"),
            title: 'Third Message',
            description: 'Item 3',
            remindTime: { date: '2020-09-28', time: '9:50:21 pm' },
            isComplete: false,
        }

const parseDate1= remindTime.remindTime.date.split('-');
//this returns [2021,09,28]
const parseDate2= remindTime2.remindTime.date.split('-');
//this returns [2021,09,26]

var a = moment(parseDate1);
var b = moment(parseDate2);

//then you can compare dates in different ways sush us:

a.diff(b, 'days')   // 1
a.diff(b) // 86400000 => this is miliseconds
a.diff(b, 'years');   // 1

Also it could be easier to think that diff is a kind of minus operator:

a.diff(b) // a - b < 0
b.diff(a) // b - a > 0

Hope it works, for further info please check this out: https://momentjs.com/docs/#/query/