What Are The Differences Between The HourCycle Options In Date.prototype.toLocaleTimeString()

MDN documentation for toLocaleTimeString() indicates that the hourCycle and hc options have four possible values: 'h11', 'h12', 'h23', & 'h24'. Two of the possible values strik

Solution 1:

I found that the proposal of dateStyle and timeStyle options for Intl.DateTimeFormat says:

[[HourCycle]] is a String value indicating whether the 12-hour format ("h11", "h12") or the 24-hour format ("h23", "h24") should be used. "h11" and "h23" start with hour 0 and go up to 11 and 23 respectively. "h12" and "h24" start with hour 1 and go up to 12 and 24. [[HourCycle]] is only used when [[Hour]] is not undefined.

English or US style may prefer h12:

› new Date(2019,4,1,12,0,0).toLocaleString('en-US', { hourCycle: 'h12' })
‹ "5/1/2019, 12:00:00 PM"
› new Date(2019,4,1,12,0,0).toLocaleString('en-US', { hourCycle: 'h11' })
‹ "5/1/2019, 0:00:00 PM"

h24 must be used with caution. It would have been nice if the date part was the value one day before.

› new Date(2019,4,1,0,59,59).toLocaleString('ja-JP', { hourCycle: 'h23' })
‹ "2019/5/1 0:59:59"
› new Date(2019,4,1,0,59,59).toLocaleString('ja-JP', { hourCycle: 'h24' })
‹ "2019/5/1 24:59:59"

Compatibility table in MDN says Firefox 58 and Edge supports this.

Solution 2:

I agree it's currently difficult to find MDN's documentation on hourCycle values, but I found them here:

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Locale/hourCycle

Web technology for developers > JavaScript > JavaScript reference > Standard built-in objects > Intl.Locale > Intl.Locale.prototype.hourCycle

[…]

h12: Hour system using 1–12; corresponds to 'h' in patterns. The 12 hour clock, with midnight starting at 12:00 am.

h23: Hour system using 0–23; corresponds to 'H' in patterns. The 24 hour clock, with midnight starting at 0:00.

h11: Hour system using 0–11; corresponds to 'K' in patterns. The 12 hour clock, with midnight starting at 0:00 am.

h24: Hour system using 1–24; corresponds to 'k' in pattern. The 24 hour clock, with midnight starting at 24:00.