How do computers figure out date information?

Question

Most languages have some sort of date function where you really don't have to do any programming to get any date information you just get it from the date function/object. I am curious what goes on behind the scenes to make this happen?

Answer 1

Every computer has a system clock which keeps track of date and time. On the lowest level, date and time information it retrieved from there. Above that add timezone information, etc. from the operating system and you got a Date object or something similar.

Depending on your language/environment Date objects can either perform date calculation themselves or you have to use other functions to achieve that. Those ensure that leap years get handled correctly and no invalid date can be created.

But maybe I got your question wrong.

Answer 2

Date and time information is provided usually by operating system, so it's a system call. Operating system deals with realtime clock mounted on computer mainboard and powered by small battery (which lasts for years).

Answer 3

Typically a computer is storing a count of how many of a certain unit of time has gone by since a specific time and date in the past. In Unix systems, for example, this could be the number of seconds since the Unix Epoch, which is midnight, Jan 1st 1970 GMT. In Windows, this is the number of 100 ns intervals since 1601-01-0 (thanks Johannes Rössel). Or it could be something as simple as number of seconds since the computer was powered on.

So from the number of units that have gone by since that time/date, an OS can calculate the number of years, months, days, etc that have gone by. Of course all sorts of fun stuff like leap years and leap seconds have to be taken into account for this to occur.

Systems such as NTP (Network Time Protocol) can be used to synchronize a computer's internal count to atomic clocks via an NTP server over a network. To do this, they NTP takes into account the round trip time and learns the sorts of errors the link to the NTP server.

Answer 4

Your computer has a system clock and the BIOS has a timer function that can be updated from your OS. Languages only take the information from there and some can update it too.

Answer 5

Well ... Most computers contain a "real-time clock", which counts time on the human scale of seconds, minutes etc. Traditionally, there is a small battery on the motherboard, that lets the chip either remember the time, or even keep counting it, even when the rest of the computer is powered off.

Many computers today use services like the network time protocol to periodically query a centralized high-precision clock, to set the current time. In this way, even if the battery is removed (or just fails), the computer will still know what time and date it is, and be able to update (to correct for errors in the real-time chip's time-keeping) that information as often as necessary.

Answer 6

The date can be synchronized using ntpdate command via NTP

The following is the linix man page of the ntpdate command

DESCRIPTION
   ntpdate  sets  the local date and time by polling the Network Time Protocol (NTP) server(s) given as the server
   arguments to determine the correct time. It must be run as root on the local host  (unless  the  option  -q  is
   used). A number of samples are obtained from each of the servers specified and a subset of the NTP clock filter
   and selection algorithms are applied to select the best of these. Note that the  accuracy  and  reliability  of
   ntpdate  depends  on  the  number  of servers, the number of polls each time it is run and the interval between
   runs.

   ntpdate can be run manually as necessary to set the host clock, or it can be run from the host  startup  script
   to set the clock at boot time.  This is useful in some cases to set the clock initially before starting the NTP
   daemon ntpd. It is also possible to run ntpdate from a cron script. However, it is important to note that  ntp‐
   date  with  contrived  cron scripts is no substitute for the NTP daemon, which uses sophisticated algorithms to
   maximize accuracy and reliability while minimizing resource use. Finally, since ntpdate does not discipline the
   host clock frequency as does ntpd, the accuracy using ntpdate is limited.

   Time  adjustments are made by ntpdate in one of two ways. If ntpdate determines the clock is in error more than
   0.5 second it will simply step the time by calling the system settimeofday() routine. If the error is less than
   0.5  seconds,  it will slew the time by calling the system adjtime() routine. The latter technique is less dis‐
   ruptive and more accurate when the error is small, and works quite well when ntpdate is run by cron every  hour
   or two.

   ntpdate  will  decline  to  set the date if an NTP server daemon (e.g., ntpd) is running on the same host. When
   running ntpdate on a regular basis from cron as an alternative to running a daemon, doing so once every hour or
   two will result in precise enough timekeeping to avoid stepping the clock.

Answer 7

Buy any of these books on Calendrical Calculations. They'll fill you in on how the date libraries work under the hood.

Answer 8

The date/time is often stored in terms of times since a certain date. For example ticks (100 nanosecond intervals) since January 1, 0001. It is also usueally stored in reference to UTC. The underlying methods in the OS, database, framework, application, etc. can then convert these to a more usable representation. Back in the day, systems would store component parts of the date, day, month, year, etc as a part of the data structure, but we learned our lesson with the Y2K mess that this probably isn't the best approach.

Answer 9

A surprising amount of surprisingly complicated code is required for date parsing, computation, creation etc.

For example, in Java, dates are computed, modified, stored etc via the Date, Calendar, and specifically and typically, the Gregorian Calendar implementation of Calendar. (You can download the SDK/JDK and look at the source for yourself.)

In short, what I took from a quick perusal of the source is: Date handling is non-trivial and not something you want to attempt on your own. Find a good library if at all possible, else you will almost certainly be reinventing the square wheel.

Answer 10

Most replies have been to do with how the current date is obtained. i.e. from system clock and so on.

If you want to know how it is stored and used there are many different implementations and it depends on the system.

I believe a common one is the use of a 64 bit signed integer in T-sql the 01/01/1970 is 0 so negative numbers are pre 1970 and positive on from that each increment adding 100 th of a second (think it's a 100th would need to check).

Why 01/01/1970 you may ask this is because the gregorian calendar is on a 400 year cycle. 01/01/1970 being the closes start of a cycle to the current date.

This is because "Every year that is exactly divisible by four is a leap year, except for years that are exactly divisible by 100; the centurial years that are exactly divisible by 400 are still leap years. For example, the year 1900 is not a leap year; the year 2000 is a leap year." Makes it very complicated I believe the 400 year cycle coincides with the days of the week repeating as well but would nee dto check. Basically it's very complicated.

Internally it is incredibly difficult to write the datetime library accounting for all these variations such as leap years, the fact there is no year zero..... Not to mention UTC, GMT UT1 times.

Answer 11

We had occasion when debugging a client problem to look at how SQL stores datetimes... fairly interesting and makes pretty good sense once you see it.

SQL uses 2 4 byte integers... The first 4 bytes are the date in days since Jan. 1st, 1753. I believe the maximum year is supposed to be 9999, which doesn't exactly line up to the number of available integers in 4 bytes, but there you go. The second 4 bytes are the time in milliseconds since midnight.