moment().startOf('year'); // set to January 1st, 12:00 am this year
moment().startOf('month'); // set to the first of this month, 12:00 am
moment().startOf('quarter'); // set to the beginning of the current quarter, 1st day of months, 12:00 am
moment().startOf('week'); // set to the first day of this week, 12:00 am
moment().startOf('isoWeek'); // set to the first day of this week according to ISO 8601, 12:00 am
moment().startOf('day'); // set to 12:00 am today
moment().startOf('date'); // set to 12:00 am today
moment().startOf('hour'); // set to now, but with 0 mins, 0 secs, and 0 ms
moment().startOf('minute'); // set to now, but with 0 seconds and 0 milliseconds
moment().startOf('second'); // same as moment().milliseconds(0);