How to get UTC 0 o'clock of current date in javascript

It's common that we need to get the UTC date of today. With Javascript, you can get the current day easily with new Date() or Date.now(). 

But sometimes, we would like to treat any time in the same day the same way, which means we don't want to know the hour, minute, second of the day. We just care about the day. 

You can always convert the date to string and ignore the hours etc. which is inefficient. It's also problematical if we need to save the date into db. So convert to string then convert back to time stamp is necessary but make it more inefficient.

Today I am showing a pure mathematical way.

TodayUTCZeroOClock = Now - Now%DayInMiniSeconds

Example:

a = Date.now()

1552561275091

day = 1000 * 60　* 60 * 24

86400000

a%day

39675091

b = a - a%day

1552521600000

new Date(b)

Thu Mar 14 2019 13:00:00 GMT+1300 (New Zealand Daylight Time)

Note that 13 O'clock is UTC 0 0'clock