Returns a cloned Day.js object and set it to the start of a unit of time.
Units are case insensitive, and support plural and short forms.
List of all available units
|January 1st, 00:00 this year|
|beginning of the current quarter, 1st day of months, 00:00 (
|the first day of this month, 00:00|
|the first day of this week, 00:00|
|the first day of this week according to ISO 8601, 00:00 (
|now, but with 0 mins, 0 secs, and 0 ms|
|now, but with 0 seconds and 0 milliseconds|
|now, but with 0 milliseconds|