moment startOf() and endOf() is the answer you are searching for.. For Example:-
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('week'); // set to the first day of this week, 12:00 am
moment().startOf('day'); // set to 12:00 am today