Your Time Machine backup disk might not always be available, so Time Machine also stores some of its backups on your Mac. These backups are called local snapshots.
US Naval Observatory Master Clock Time: Wed, 28 Apr 2021 03:34:55 UTC. World clock, time zone converters and meeting planner. Event time & date sharing. Daylight Saving Time dates. Sunrise/sunset and moonrise/moonset. Now a web application.
How to use local snapshots
When your backup disk isn't available, Time Machine automatically uses local snapshots to help you restore files. Reconnect your backup disk to make even more backups available.
Learn how to restore files from Time Machine.
How local snapshots use storage space
You don't need to think about how much storage space local snapshots are using, because they don't use space needed for tasks like downloading files, copying files, or installing new software.
Your Mac counts the space used by snapshots as available storage. Even so, Time Machine stores snapshots only on disks that have plenty of free space, and it automatically deletes snapshots as they age or as space is needed for other things.
If you want to delete local snapshots manually, turn off Time Machine temporarily:
- Open Time Machine preferences from the Time Machine menu in the menu bar. Or choose Apple menu > System Preferences, then click Time Machine.
- Deselect ”Back Up Automatically” or click the Off/On switch, depending on what you see in Time Machine preferences.
- Wait a few minutes to allow the local snapshots to be deleted. Then turn on Time Machine again. It remembers your backup disks.
How often local snapshots are saved
Time Machine saves one snapshot of your startup disk approximately every hour, and keeps it for 24 hours. It keeps an additional snapshot of your last successful Time Machine backup until space is needed. And in macOS High Sierra or later, another snapshot is saved before installing any macOS update.
Normally time is shown as Hours:Minutes
There are 24 Hours in a Day and 60 Minutes in each Hour.
Showing the Time
Time With Seconds
There are two main ways to show the time: '24 Hour Clock' or 'AM/PM':
24 Hour Clock: the time is shown as how many hours and minutes since midnight.
AM/PM (or '12 Hour Clock'): the day is split into: - the 12 Hours running from Midnight to Noon (the AM hours), and
- the other 12 Hours running from Noon to Midnight (the PM hours).
Like this (try the slider):
PM | ||
Post Meridiem* Latin for 'after midday' | ||
Midnight to Noon | ||
24 Hour Clock: | 12:00 to 23:59 |
Time Zones
*Is that spelled 'Meridiem' or 'Meridian'? See here.
Converting AM/PM to 24 Hour Clock
Add 12 to any hour after Noon (and subtract 12 for the first hour of the day):
For the first hour of the day (12 Midnight to 12:59 AM), subtract 12 Hours
From 1:00 AM to 12:59 PM, no change
Examples: 11:20 AM = 11:20, 12:30 PM = 12:30
From 1:00 PM to 11:59 PM, add 12 Hours
Converting 24 Hour Clock to AM/PM
For the first hour of the day (00:00 to 00:59), add 12 Hours, make it 'AM'
Time Now
Examples: 00:10 = 12:10 AM, 00:40 = 12:40 AM
From 01:00 to 11:59, just make it 'AM'
From 12:00 to 12:59, just make it 'PM'
Examples: 12:10 = 12:10 PM, 12:55 = 12:55 PM
From 13:00 to 23:59, subtract 12 Hours, make it 'PM'
Comparison Chart
Here is a side-by-side comparison of the 24 Hour Clock and AM/PM:Example: 10 minutes past | ||||
| AM / PM | 24 Hour Clock | AM / PM | ||
| 00:00 | 12 Midnight | 00:10 | 12:10 AM | |
| 01:00 | 1:00 AM | 01:10 | 1:10 AM | |
| 02:00 | 2:00 AM | 02:10 | 2:10 AM | |
| 03:00 | 3:00 AM | 03:10 | 3:10 AM | |
| 04:00 | 4:00 AM | 04:10 | 4:10 AM | |
| 05:00 | 5:00 AM | 05:10 | 5:10 AM | |
| 06:00 | 6:00 AM | 06:10 | 6:10 AM | |
| 07:00 | 7:00 AM | 07:10 | 7:10 AM | |
| 08:00 | 8:00 AM | 08:10 | 8:10 AM | |
| 09:00 | 9:00 AM | 09:10 | 9:10 AM | |
| 10:00 | 10:00 AM | 10:10 | 10:10 AM | |
| 11:00 | 11:00 AM | 11:10 | 11:10 AM | |
| 12:00 | 12 Noon | 12:10 | 12:10 PM | |
| 13:00 | 1:00 PM | 13:10 | 1:10 PM | |
| 14:00 | 2:00 PM | 14:10 | 2:10 PM | |
| 15:00 | 3:00 PM | 15:10 | 3:10 PM | |
| 16:00 | 4:00 PM | 16:10 | 4:10 PM | |
| 17:00 | 5:00 PM | 17:10 | 5:10 PM | |
| 18:00 | 6:00 PM | 18:10 | 6:10 PM | |
| 19:00 | 7:00 PM | 19:10 | 7:10 PM | |
| 20:00 | 8:00 PM | 20:10 | 8:10 PM | |
| 21:00 | 9:00 PM | 21:10 | 9:10 PM | |
| 22:00 | 10:00 PM | 22:10 | 10:10 PM | |
| 23:00 | 11:00 PM | 23:10 | 11:10 PM | |
Midnight and Noon
'12 AM' and '12 PM' can cause confusion, so we prefer '12 Midnight' and '12 Noon'.
What Day is Midnight?
Midnight has another problem: there is nothing to tell us 'is this the beginning or ending of the day'.
Imagine your friends say they are leaving for holiday at 'midnight' on 12th March, what day should you arrive to say goodbye?
Do you get there on the 11th (assuming they leave at the very start of the 12th), or the 12th (assuming they leave at the end of the 12th)?
It is better to use:
- 11:59 PM or 12:01 AM, or
- 23:59 or 00:01 (24-Hour Clock)
which the railroads, airlines and military actually do.
So, when you see something like 'offer ends midnight October 15th' tell them to use one minute before or after so there is no confusion!
Footnote on 'Meridiem' vs 'Meridian'
Should 'AM' be 'ante meridiem' or 'ante meridian' (likewise for PM)?
The official (according to an American, Australian and British dictionary I checked), and most common spelling for AM is 'ante meridiem' which is a Latin phrase. I recommend that spelling!
But people sometimes use the phrase 'ante meridian' (a 'meridian' in this case refers to an imaginary line in the sky when the sun is at its highest point).