<s>
In	O
computer	B-General_Concept
science	I-General_Concept
and	O
computer	B-General_Concept
programming	I-General_Concept
,	O
system	B-Operating_System
time	I-Operating_System
represents	O
a	O
computer	O
system	O
's	O
notion	O
of	O
the	O
passage	O
of	O
time	B-Device
.	O
</s>
<s>
In	O
this	O
sense	O
,	O
time	B-Device
also	O
includes	O
the	O
passing	O
of	O
days	O
on	O
the	O
calendar	B-Application
.	O
</s>
<s>
System	B-Operating_System
time	I-Operating_System
is	O
measured	O
by	O
a	O
system	B-Operating_System
clock	I-Operating_System
,	O
which	O
is	O
typically	O
implemented	O
as	O
a	O
simple	O
count	O
of	O
the	O
number	O
of	O
ticks	O
that	O
have	O
transpired	O
since	O
some	O
arbitrary	O
starting	O
date	O
,	O
called	O
the	O
epoch	O
.	O
</s>
<s>
For	O
example	O
,	O
Unix	B-Application
and	O
POSIX-compliant	O
systems	O
encode	O
system	B-Operating_System
time	I-Operating_System
(	O
"	O
Unix	B-Operating_System
time	I-Operating_System
"	O
)	O
as	O
the	O
number	O
of	O
seconds	O
elapsed	O
since	O
the	O
start	O
of	O
the	O
Unix	B-Operating_System
epoch	I-Operating_System
at	O
1	B-Operating_System
January	I-Operating_System
1970	I-Operating_System
00:00:00	O
UT	O
,	O
with	O
exceptions	O
for	O
leap	O
seconds	O
.	O
</s>
<s>
Systems	O
that	O
implement	O
the	O
32-bit	O
and	O
64-bit	O
versions	O
of	O
the	O
Windows	B-Library
API	I-Library
,	O
such	O
as	O
Windows	B-Application
9x	O
and	O
Windows	B-Device
NT	I-Device
,	O
provide	O
the	O
system	B-Operating_System
time	I-Operating_System
as	O
both	O
,	O
represented	O
as	O
a	O
year/month/day/hour/minute/second/milliseconds	O
value	O
,	O
and	O
,	O
represented	O
as	O
a	O
count	O
of	O
the	O
number	O
of	O
100-nanosecond	O
ticks	O
since	O
1	O
January	O
1601	O
00:00:00	O
UT	O
as	O
reckoned	O
in	O
the	O
proleptic	O
Gregorian	O
calendar	B-Application
.	O
</s>
<s>
System	B-Operating_System
time	I-Operating_System
can	O
be	O
converted	O
into	O
calendar	B-Application
time	B-Device
,	O
which	O
is	O
a	O
form	O
more	O
suitable	O
for	O
human	O
comprehension	O
.	O
</s>
<s>
For	O
example	O
,	O
the	O
Unix	B-Application
system	I-Application
time	B-Device
seconds	O
since	O
the	O
beginning	O
of	O
the	O
epoch	O
translates	O
into	O
the	O
calendar	B-Application
time	B-Device
9	O
September	O
2001	O
01:46:40	O
UT	O
.	O
</s>
<s>
Library	O
subroutines	O
that	O
handle	O
such	O
conversions	O
may	O
also	O
deal	O
with	O
adjustments	O
for	O
time	B-Device
zones	I-Device
,	O
daylight	O
saving	O
time	B-Device
(	O
DST	O
)	O
,	O
leap	O
seconds	O
,	O
and	O
the	O
user	O
's	O
locale	B-Device
settings	O
.	O
</s>
<s>
Library	O
routines	O
are	O
also	O
generally	O
provided	O
that	O
convert	O
calendar	B-Application
times	O
into	O
system	B-Operating_System
times	I-Operating_System
.	O
</s>
<s>
Closely	O
related	O
to	O
system	B-Operating_System
time	I-Operating_System
is	O
process	B-Operating_System
time	B-Device
,	O
which	O
is	O
a	O
count	O
of	O
the	O
total	O
CPU	B-General_Concept
time	B-Device
consumed	O
by	O
an	O
executing	O
process	B-Operating_System
.	O
</s>
<s>
It	O
may	O
be	O
split	O
into	O
user	O
and	O
system	O
CPU	B-General_Concept
time	B-Device
,	O
representing	O
the	O
time	B-Device
spent	O
executing	O
user	O
code	O
and	O
system	O
kernel	B-Operating_System
code	O
,	O
respectively	O
.	O
</s>
<s>
Process	B-Operating_System
times	O
are	O
a	O
tally	O
of	O
CPU	B-General_Concept
instructions	B-General_Concept
or	O
clock	O
cycles	O
and	O
generally	O
have	O
no	O
direct	O
correlation	O
to	O
wall	B-General_Concept
time	I-General_Concept
.	O
</s>
<s>
File	B-Application
systems	I-Application
keep	O
track	O
of	O
the	O
times	O
that	O
files	O
are	O
created	O
,	O
modified	O
,	O
and/or	O
accessed	O
by	O
storing	O
timestamps	O
in	O
the	O
file	B-Operating_System
control	I-Operating_System
block	I-Operating_System
(	O
or	O
inode	B-Application
)	O
of	O
each	O
file	B-Operating_System
and	O
directory	B-Application
.	O
</s>
<s>
Most	O
first-generation	O
personal	B-Device
computers	I-Device
did	O
not	O
keep	O
track	O
of	O
dates	O
and	O
times	O
.	O
</s>
<s>
These	O
included	O
systems	O
that	O
ran	O
the	O
CP/M	B-Application
operating	I-Application
system	I-Application
,	O
as	O
well	O
as	O
early	O
models	O
of	O
the	O
Apple	B-Device
II	I-Device
,	O
the	O
BBC	B-Device
Micro	I-Device
,	O
and	O
the	O
Commodore	B-Device
PET	I-Device
,	O
among	O
others	O
.	O
</s>
<s>
Add-on	O
peripheral	O
boards	O
that	O
included	O
real-time	O
clock	O
chips	O
with	O
on-board	O
battery	O
back-up	O
were	O
available	O
for	O
the	O
IBM	B-Device
PC	I-Device
and	O
XT	B-Device
,	O
but	O
the	O
IBM	B-Operating_System
AT	I-Operating_System
was	O
the	O
first	O
widely	O
available	O
PC	O
that	O
came	O
equipped	O
with	O
date/time	O
hardware	O
built	O
into	O
the	O
motherboard	B-Device
.	O
</s>
<s>
Prior	O
to	O
the	O
widespread	O
availability	O
of	O
computer	B-Architecture
networks	I-Architecture
,	O
most	O
personal	B-Device
computer	I-Device
systems	O
that	O
did	O
track	O
system	B-Operating_System
time	I-Operating_System
did	O
so	O
only	O
with	O
respect	O
to	O
local	O
time	B-Device
and	O
did	O
not	O
make	O
allowances	O
for	O
different	O
time	B-Device
zones	I-Device
.	O
</s>
<s>
With	O
current	O
technology	O
,	O
most	O
modern	O
computers	O
keep	O
track	O
of	O
local	O
civil	O
time	B-Device
,	O
as	O
do	O
many	O
other	O
household	O
and	O
personal	O
devices	O
such	O
as	O
VCRs	O
,	O
DVRs	O
,	O
cable	O
TV	O
receivers	O
,	O
PDAs	B-Application
,	O
pagers	B-Application
,	O
cell	O
phones	O
,	O
fax	O
machines	O
,	O
telephone	O
answering	O
machines	O
,	O
cameras	O
,	O
camcorders	O
,	O
central	O
air	O
conditioners	O
,	O
and	O
microwave	O
ovens	O
.	O
</s>
<s>
Microcontrollers	B-Architecture
operating	O
within	O
embedded	B-Architecture
systems	I-Architecture
(	O
such	O
as	O
the	O
Raspberry	B-Operating_System
Pi	I-Operating_System
,	O
Arduino	O
,	O
and	O
other	O
similar	B-Device
systems	I-Device
)	O
do	O
not	O
always	O
have	O
internal	O
hardware	O
to	O
keep	O
track	O
of	O
time	B-Device
.	O
</s>
<s>
Many	O
such	O
controller	O
systems	O
operate	O
without	O
knowledge	O
of	O
the	O
external	O
time	B-Device
.	O
</s>
<s>
Those	O
that	O
require	O
such	O
information	O
typically	O
initialize	O
their	O
base	O
time	B-Device
upon	O
rebooting	B-Device
by	O
obtaining	O
the	O
current	O
time	B-Device
from	O
an	O
external	O
source	O
,	O
such	O
as	O
from	O
a	O
time	B-Protocol
server	I-Protocol
or	O
external	O
clock	O
,	O
or	O
by	O
prompting	B-Application
the	O
user	O
to	O
manually	O
enter	O
the	O
current	O
time	B-Device
.	O
</s>
<s>
The	O
system	B-Operating_System
clock	I-Operating_System
is	O
typically	O
implemented	O
as	O
a	O
programmable	B-Device
interval	I-Device
timer	I-Device
that	O
periodically	O
interrupts	O
the	O
CPU	B-General_Concept
,	O
which	O
then	O
starts	O
executing	O
a	O
timer	O
interrupt	O
service	O
routine	O
.	O
</s>
<s>
This	O
routine	O
typically	O
adds	O
one	O
tick	O
to	O
the	O
system	B-Operating_System
clock	I-Operating_System
(	O
a	O
simple	O
counter	O
)	O
and	O
handles	O
other	O
periodic	O
housekeeping	O
tasks	O
(	O
preemption	B-Operating_System
,	O
etc	O
.	O
)	O
</s>
<s>
before	O
returning	O
to	O
the	O
task	O
the	O
CPU	B-General_Concept
was	O
executing	O
before	O
the	O
interruption	O
.	O
</s>
<s>
The	O
following	O
tables	O
illustrate	O
methods	O
for	O
retrieving	O
the	O
system	B-Operating_System
time	I-Operating_System
in	O
various	O
operating	B-General_Concept
systems	I-General_Concept
,	O
programming	O
languages	O
,	O
and	O
applications	B-Application
.	O
</s>
<s>
All	O
dates	O
are	O
given	O
as	O
Gregorian	O
or	O
proleptic	O
Gregorian	O
calendar	B-Application
dates	O
.	O
</s>
<s>
Note	O
that	O
the	O
resolution	O
of	O
an	O
implementation	O
's	O
measurement	O
of	O
time	B-Device
does	O
not	O
imply	O
the	O
same	O
precision	O
of	O
such	O
measurements	O
.	O
</s>
<s>
For	O
example	O
,	O
a	O
system	O
might	O
return	O
the	O
current	O
time	B-Device
as	O
a	O
value	O
measured	O
in	O
microseconds	O
,	O
but	O
actually	O
be	O
capable	O
of	O
discerning	O
individual	O
clock	B-Operating_System
ticks	I-Operating_System
with	O
a	O
frequency	O
of	O
only	O
100Hz	O
(	O
10ms	O
)	O
.	O
</s>
<s>
Operating	B-General_Concept
system	I-General_Concept
Command	O
or	O
function	O
Resolution	O
Epoch	O
or	O
rangeAndroid	O
1	O
ms1	O
January	O
1970BIOS	O
(	O
IBM	B-Device
PC	I-Device
)	O
54.9254	O
ms	B-Language
18.2065	O
Hz	O
Midnight	O
of	O
the	O
current	O
day	O
1	O
s	O
Midnight	O
of	O
the	O
current	O
day	O
1	O
day	O
1	O
January	O
1980	O
to	O
31	O
December	O
1999	O
or	O
31	O
December	O
2079	O
(	O
system	O
dependent	O
)	O
CP/M	B-Operating_System
Plus	I-Operating_System
System	O
Control	O
Block	O
:	O
,	O
Days	O
since	O
31	O
December	O
1977	O
,	O
Hour	O
(	O
BCD	O
)	O
,	O
Minute	O
(	O
BCD	O
)	O
,	O
Second	O
(	O
BCD	O
)	O
1	O
s	O
31	O
December	O
1977	O
to	O
5	O
June	O
2157	O
BDOS	O
function	O
>	O
(	O
)	O
:	O
,	O
Days	O
since	O
1	O
January	O
1978	O
,	O
Hour	O
(	O
BCD	O
)	O
,	O
Minute	O
(	O
BCD	O
)	O
,	O
Second	O
(	O
BCD	O
)	O
DOS	B-Device
(	O
Microsoft	O
)	O
DATETIME	O
10	O
ms	B-Language
1	O
January	O
1980	O
to	O
31	O
December	O
2099	O
INT	O
21h	O
,	O
AH	O
=	O
2Ch	O
SYSTEM	O
TIMEINT	O
21h	O
,	O
AH	O
=	O
2Ah	O
SYSTEM	B-Operating_System
DATE	I-Operating_System
iOS	B-Application
(	O
Apple	O
)	O
CFAbsoluteTimeGetCurrent( )	O
<	O
1	O
ms	B-Language
1	O
January	O
2001	O
±	O
10,000	O
years	O
macOS	B-Application
CFAbsoluteTimeGetCurrent( )	O
<	O
1	O
msThe	O
Apple	O
Developer	O
Documentation	O
is	O
not	O
clear	O
on	O
the	O
precision	O
&	O
range	O
of	O
CFAbsoluteTime/CFTimeInterval	O
,	O
except	O
in	O
the	O
CFRunLoopTimerCreate	O
documentation	O
which	O
refers	O
to	O
'	O
sub-millisecond	O
at	O
most	O
 '	O
precision	O
.	O
</s>
<s>
1	O
January	O
2001	O
±	O
10,000	O
yearsOpenVMS	O
SYS$	O
GETTIM( )	O
100	O
ns	O
17	O
November	O
1858	O
to	O
31	O
July	O
31,086	O
(	O
)	O
1	O
μs1	O
January	O
1970	O
to	O
7	O
February	O
2106	O
(	O
)	O
1	O
ns	O
z/OS	B-Application
STCK2−12	O
μs	O
244.14	O
ps	O
1	O
January	O
1900	O
to	O
17	O
September	O
2042	O
UTIBM	O
intends	O
to	O
extend	O
the	O
date	O
range	O
on	O
future	O
systems	O
beyond	O
2042	O
.	O
z/Architecture	O
Principles	O
of	O
Operation	O
,	O
(	O
Poughkeepsie	O
,	O
New	O
York:International	O
Business	O
Machines	O
,	O
2007	O
)	O
1-15	O
,	O
4-45	O
to	O
4-47	O
.	O
</s>
<s>
Language/ApplicationFunction	O
or	O
variableResolutionEpoch	O
or	O
range	O
Ada	B-Language
Ada.Calendar.Clock	O
100	O
μs	O
to	O
20	O
ms	B-Language
( *	O
)	O
1	O
January	O
1901	O
to	O
31	O
December	O
2099	O
( *	O
)	O
AWK	B-Application
systime( )	O
1	O
s	O
( *	O
)	O
BASIC	O
,	O
True	B-Language
BASIC	I-Language
DATE	O
,	O
DATE$	O
TIME	B-Device
,	O
TIME$	O
1	O
s	O
( *	O
)	O
Business	B-Language
BASIC	I-Language
DAY	O
,	O
TIM	O
0.1	O
s	O
( *	O
)	O
C	B-Language
(	O
see	O
C	B-Language
date	I-Language
and	I-Language
time	I-Language
functions	I-Language
)	O
time( )	O
1	O
s	O
( *	O
)	O
The	B-Language
C	I-Language
standard	I-Language
library	I-Language
does	O
not	O
specify	O
any	O
specific	O
resolution	O
,	O
epoch	O
,	O
range	O
,	O
or	O
datatype	O
for	O
system	B-Operating_System
time	I-Operating_System
values	O
.	O
</s>
<s>
The	O
C++	B-Language
library	O
encompasses	O
the	O
C	B-Language
library	I-Language
,	O
so	O
it	O
uses	O
the	O
same	O
system	B-Operating_System
time	I-Operating_System
implementation	O
as	O
C	B-Language
.	O
( *	O
)	O
C++	B-Language
std::time( )	O
std::chrono::system_clock::now( )	O
1	O
s	O
( *	O
)	O
1	O
ns	O
(	O
C++11	O
,	O
OS	B-General_Concept
dependent	O
)	O
( *	O
)	O
C#	B-Application
System.DateTime.Now	O
System.DateTime.UtcNow	O
100	O
ns	O
1	O
January	O
0001	O
to	O
31	O
December	O
9999	O
CICS	B-Application
ASKTIME	O
1	O
ms	B-Language
1	O
January	O
1900	O
COBOL	B-Application
FUNCTION	O
CURRENT-DATE	O
1	O
s	O
1	O
January	O
1601	O
Common	B-Language
Lisp	I-Language
(	O
get-universal-time	O
)	O
1	O
s	O
1	O
January	O
1900	O
Delphi	B-Language
(	O
Borland	O
)	O
datetime	O
1	O
ms	B-Language
(	O
floating	O
point	O
)	O
1	O
January	O
1900	O
Delphi	B-Language
(	O
Embarcadero	O
Technologies	O
)	O
System.SysUtils.Time	O
1	O
ms	B-Language
0/0/0000	O
0:0:0:000	O
to	O
12/31/9999	O
23:59:59:999	O
 [ sic ] 	O
System.SysUtils.GetTime	O
(	O
alias	O
for	O
System.SysUtils.Time	O
)	O
System.SysUtils.Date	O
0/0/0000	O
0:0:0:000	O
to	O
12/31/9999	O
0:0:0:000	O
 [ sic ] 	O
System.DateUtils.Today	O
System.DateUtils.Tomorrow	O
System.DateUtils.Yesterday	O
System.SysUtils.Now	O
1	O
s	O
0/0/0000	O
0:0:0:000	O
to	O
12/31/9999	O
23:59:59:000	O
 [ sic ] 	O
System.SysUtils.DayOfWeek	O
1	O
day	O
1	O
to	O
7	O
System.SysUtils.CurrentYear	O
1	O
year	O
( *	O
)	O
Emacs	B-Operating_System
Lisp	I-Operating_System
(	O
current-time	O
)	O
1	O
μs	O
( *	O
)	O
1	B-Operating_System
January	I-Operating_System
1970	I-Operating_System
Erlang	B-Operating_System
erlang:system_time( )	O
,	O
os:system_time( )	O
OS	B-General_Concept
dependent	O
,	O
e.g.	O
</s>
<s>
on	O
Linux	B-Application
1ns	O
1	B-Operating_System
January	I-Operating_System
1970	I-Operating_System
Excel	B-Application
date( )	O
?	O
</s>
