Understanding Unix time
What is Unix time?
Unix time is the number of seconds that have elapsed since midnight (00:00:00), Thursday, 1 January 1970, UTC, not including leap seconds.
If you read my piece on dealing with dates you'll know this particular date can be represented in ISO-8601 format as 1970-01-01T00:00:00Z. This reference point in time is known as the Unix Epoch.
As with many timing schemes based on computers or atomic timing devices, there is a difference between these absolute times and astronomical times. This is because astronomical times are based on the Earth's rotation and this is not exactly consistent. Discrepancies therefore creep in over time. Leap seconds are therefore required to adjust UTC to resynchronize it with astronomical time.
Since 1970 only positive leap seconds (insertions) have been added to a day, and no negative leap seconds have been required (yet).
Unix time treats each and every day as exactly 24 * 60 * 60 seconds, which is 86400 seconds, and so does not take into account leap seconds. Unix time is therefore equivalent to the value of UTC seconds since midnight 1st January 1970 minus one second for each leap second since the Epoch. Since UTC was introduced in 1972 there have been 27 leap seconds inserted into it, so, as I write this, it is:
Unix time seconds = UTC seconds (since midnight 1st January 1970) - 27
There's a slightly strange area between 1st January 1970 and 1st January 1972 because UTC wasn't introduced until the latter of those dates.
There's quite a bit of controversy around leap seconds, and how to implement them in Unix libraries, and even a move to just get rid of them completely. As far as Unix time is concerned though, to all intents you can ignore them unless you are trying to convert between Unix timestamps and UTC and vice versa. I have provided some links below though in case you are a masochist - it's gnarly stuff...
So moving swiftly on to more practical matters...
Working with dates
If you run date on the command line:
date
You would get a datetime in a human readable format:
Tue 30 Oct 2018 14:35:59 GMT
To obtain UTC time you can do:
date -u
This would return a date in the format:
Tue 30 Oct 2018 14:47:27 UTC
NOTE: In UK we just came off BST this weekend, so GMT and UTC are essentially the same thing.
You can also obtain an ISO-8601 format UTC date using:
date -u +"%Y-%m-%dT%H:%M:%SZ"
Working with Unix time
On the command line you can easily find Unix time with:
date +%s
This would result in a number such as:
1540910388
Which is the number of seconds since Unix Epoch (ignoring leap seconds).
There's also a quick bit of Python code you can run in the shell to obtain Unix time rounded to whole seconds:
python -c 'import time; print int(time.time())'
Which you can easily convert to years:
python -c 'import time; print int(time.time()) / (365 * 24 * 60 * 60)'
What's Unix time good for?
Mostly Unix time is used for timestamps on files, events, and other data (such as in databases) and is not really used for "calendar time" directly. However, there are various utilities that will convert correctly between a Unix timestamp and a UTC datetime (the web appears to be awash with them for some reason).
Because a Unix timestamp is simply a number with respect to Unix Epoch, it is easier to compare two Unix timestamps (you just find the difference in seconds) than it is to compare two ISO-8601 format dates.
Unix time also has the advantage that it is completely indifferent to timezones and daylight savings adjustments. It is simply the number of seconds since the Epoch. This is especially useful as a means of specifying time in a web app where users may be geographically distributed.
Unix time is used in timestamps in other applications too. One that we will be looking at in a forthcoming article is in timestamps for JSON Web Tokens (JWTs).
The Y2038 Problem
There's a problem with some systems where Unix time is stored in a 32-bit integer. This is due to overflow on January 19th 2038. Modern systems circumvent this issue by using a 64-bit integer for Unix time, which will probably be enough to last until all known civilizations (and a few unknown ones) fall into the final black hole.
In Y2038 I will be 76 (if still alive and kicking) and no doubt sporting a long white Unix hacker beard and grumpily shooing a multitude of grandchildren away from my carefully preserved Apple MacBook Pro (hahaha), which uses a silicon-based processor (hahahaha), with a clock speed of 2.5 GHz (hahahaha) and 16GB RAM (hahahahahahaha). They will flaunt and frolic with their neural net bio-processors, running Neural OS 12.1, while I remember when a Unix timestamp was considered a pretty neat and useful thing.
Summary
Unix time, in the form of simple timestamps, is used anywhere that an absolute measure of time since the Unix Epoch is required. Unix timestamps are independent of timezones, daylight savings adjustments, leap second adjustments, and various UTC and ISO style formatting concerns. They are easy to generate on the command line, or in code. All major programming languages have support for Unix time built in. In a forthcoming article we will look at how Unix timestamps are used in generating JWTs.