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<h1 align="center">SQL As Understood By SQLite</h1><p><a href="lang.html">[Top]</a></p><h2>Date And Time Functions</h2>

<p>
SQLite supports five date and time functions as follows:
</p>

<p>
<ol>
<li> <b>date(</b><i>timestring, modifier, modifier, ...</i><b>)</b> </li>
<li> <b>time(</b><i>timestring, modifier, modifier, ...</i><b>)</b> </li>
<li> <b>datetime(</b><i>timestring, modifier, modifier, ...</i><b>)</b> </li>
<li> <b>julianday(</b><i>timestring, modifier, modifier, ...</i><b>)</b> </li>
<li> <b>strftime(</b><i>format, timestring, modifier, modifier, ...</i><b>)</b> </li>
</ol>

<p>
All five date and time functions take a time string as an argument. 
The time string is followed by zero or more modifiers. 
The strftime() function also takes a format string as its first argument.
</p>

<p>
The date and time functions use a subset of
<a href="http://en.wikipedia.org/wiki/ISO_8601">IS0-8601</a> date and time
formats.
The date() function returns the date in this format: YYYY-MM-DD. 
The time() function returns the time as HH:MM:SS. 
The datetime() function returns "YYYY-MM-DD HH:MM:SS". 
The julianday() function returns the 
<a href="http://en.wikipedia.org/wiki/Julian_day">Julian day</a> - the
number of days since noon in Greenwich on November 24, 4714 B.C. 
(<a href="http://en.wikipedia.org/wiki/Proleptic_Gregorian_calendar">Proleptic Gregorian calendar</a>).
The strftime() routine returns the date formatted according to 
the format string specified as the first argument.
The format string supports the most common substitutions found in the 
<a href="http://opengroup.org/onlinepubs/007908799/xsh/strftime.html">strftime() function</a>
from the standard C library plus two new substitutions, %f and %J.
The following is a complete list of valid strftime() substitutions:
</p>

<blockquote>
<table border="0" cellpadding="0" cellspacing="0">
<tr><td><td width="10"><td></tr>

<tr><td> %d <td><td> day of month: 00
<tr><td> %f <td><td> fractional seconds: SS.SSS
<tr><td> %H <td><td> hour: 00-24 
<tr><td> %j <td><td> day of year: 001-366
<tr><td> %J <td><td> Julian day number
<tr><td> %m <td><td> month: 01-12
<tr><td> %M <td><td> minute: 00-59
<tr><td> %s <td><td> seconds since 1970-01-01
<tr><td> %S <td><td> seconds: 00-59
<tr><td> %w <td><td> day of week 0-6 with Sunday==0
<tr><td> %W <td><td> week of year: 00-53
<tr><td> %Y <td><td> year: 0000-9999
<tr><td> %% <td><td> %
</table>
</blockquote>

<p>
Notice that all other date and time functions can be expressed
in terms of strftime():
</p>

<blockquote>
<table border="0" cellpadding="0" cellspacing="0">
<tr><td><b>Function</b><td width="30"><td><b>Equivalent strftime()</b>
<tr><td>   date(...)      <td><td>  strftime('%Y-%m-%d', ...)
<tr><td>   time(...)      <td><td>  strftime('%H:%M:%S', ...)
<tr><td>   datetime(...)  <td><td>  strftime('%Y-%m-%d %H:%M:%S', ...)
<tr><td>   julianday(...) <td><td>  strftime('%J', ...)
</table>
</blockquote>

<p>
The only reasons for providing functions other than strftime() is
for convenience and for efficiency.
</p>

<h3>Time Strings</h3>

<p>A time string can be in any of the following formats:</p>

<ol>
<li> <i>YYYY-MM-DD</i>
<li> <i>YYYY-MM-DD HH:MM</i>
<li> <i>YYYY-MM-DD HH:MM:SS</i>
<li> <i>YYYY-MM-DD HH:MM:SS.SSS</i>
<li> <i>YYYY-MM-DD</i><b>T</b><i>HH:MM</i>
<li> <i>YYYY-MM-DD</i><b>T</b><i>HH:MM:SS</i>
<li> <i>YYYY-MM-DD</i><b>T</b><i>HH:MM:SS.SSS</i>
<li> <i>HH:MM</i>
<li> <i>HH:MM:SS</i>
<li> <i>HH:MM:SS.SSS</i>
<li> <b>now</b>
<li> <i>DDDDDDDDDD</i>
</ol>

<p>
In formats 5 through 7, the "T" is a literal character separating 
the date and the time, as required by 
<a href="http://www.w3c.org/TR/NOTE-datetime">ISO-8601</a>. 
Formats 8 through 10 that specify only a time assume a date of 
2000-01-01. Format 11, the string 'now', is converted into the 
current date and time as obtained from the xCurrentTime method
of the <a href="c3ref/vfs.html">sqlite3_vfs</a> object in use.
The 'now' argument to date and time functions always returns exactly the
same value for multiple invocations within the same <a href="c3ref/step.html">sqlite3_step()</a> call.
<a href="http://en.wikipedia.org/wiki/Coordinated_Universal_Time">Universal Coordinated Time (UTC)</a> is used. 
Format 12 is the 
<a href="http://en.wikipedia.org/wiki/Julian_day">Julian day number</a>
expressed as a floating point value.
</p>

<p>
Formats 2 through 10 may be optionally followed by a timezone indicator of the form
"<i>&#91;+-&#93;HH:MM</i>" or just "<i>Z</i>".  The date and time functions use UTC or "zulu"
time internally, and so the "Z" suffix is a no-op.  Any non-zero "HH:MM" suffix is
subtracted from the indicated date and time in order to compute zulu time.
For example, all of the following time strings are equivalent:
</p>

<blockquote>
2013-10-07 08:23:19.120<br>
2013-10-07T08:23:19.120Z<br>
2013-10-07 04:23:19.120-04:00<br>
2456572.84952685
</blockquote>

<p>
In formats 4, 7, and 10, the fractional seconds value SS.SSS can have
one or more digits following the decimal point.  Exactly three digits are
shown in the examples because only the first three digits are significant
to the result, but the input string can have fewer or more than three digits
and the date/time functions will still operate correctly.
Similarly, format 12 is shown with 10 significant digits, but the date/time
functions will really accept as many or as few digits as are necessary to
represent the Julian day number.
</p>

<h3>Modifiers</h3>

<p>The time string can be followed by zero or more modifiers that 
alter date and/or time. Each modifier
is a transformation that is applied to the time value to its left.
Modifiers are applied from left to right; order is important.
The available modifiers are as follows.</p>

<ol>
<li> NNN days
<li> NNN hours
<li> NNN minutes
<li> NNN.NNNN seconds
<li> NNN months
<li> NNN years
<li> start of month
<li> start of year
<li> start of day
<li> weekday N
<li> unixepoch
<li> localtime
<li> utc 
</ol>

<p>The first six modifiers (1 through 6) 
simply add the specified amount of time to the date and time 
specified by the preceding timestring and modifiers.
Note that "&plusmn;NNN months" works by rendering the original date into
the YYYY-MM-DD format, adding the &plusmn;NNN to the MM month value, then
normalizing the result.  Thus, for example, the data 2001-03-31 modified
by '+1 month' initially yields 2001-04-31, but April only has 30 days
so the date is normalized to 2001-05-01.  A similar effect occurs when
the original date is February 29 of a leapyear and the modifier is
&plusmn;N years where N is not a multiple of four.</p>

<p>The "start of" modifiers (7 through 9) shift the date backwards 
to the beginning of the current month, year or day.</p>

<p>The "weekday" modifier advances the date forward to the next date 
where the weekday number is N. Sunday is 0, Monday is 1, and so forth.</p>

<p>The "unixepoch" modifier (11) only works if it immediately follows 
a timestring in the DDDDDDDDDD format. 
This modifier causes the DDDDDDDDDD to be interpreted not 
as a Julian day number as it normally would be, but as
<a href="http://en.wikipedia.org/wiki/Unix_time">Unix Time</a> - the 
number of seconds since 1970.  If the "unixepoch" modifier does not
follow a timestring of the form DDDDDDDDDD which expresses the number
of seconds since 1970 or if other modifiers
separate the "unixepoch" modifier from prior DDDDDDDDDD then the
behavior is undefined.
Due to precision limitations imposed by the implementations use
of 64-bit integers, the "unixepoch" modifier only works for
dates between 0000-01-01 00:00:00 and 5352-11-01 10:52:47 (unix times
of -62167219200 through 10675199167).</p>

<p>The "localtime" modifier (12) assumes the time string to its left is in
Universal Coordinated Time (UTC) and adjusts the time
string so that it displays localtime.  If "localtime"
follows a time that is not UTC, then the behavior is undefined.
The "utc" is the opposite of "localtime".  "utc" assumes that the string
to its left is in the local timezone and adjusts that string to be in UTC.
If the prior string is not in localtime, then the result of "utc" is
undefined.</p>

<h3>Examples</h3>

<p>Compute the current date.<p>

<blockquote>SELECT date('now');</blockquote>

<p>Compute the last day of the current month.</p>

<blockquote>SELECT date('now','start of month','+1 month','-1 day');
</blockquote>

<p>Compute the date and time given a unix timestamp 1092941466.</p>

<blockquote>
    SELECT datetime(1092941466, 'unixepoch');
</blockquote>

<p>Compute the date and time given a unix timestamp 1092941466, and 
compensate for your local timezone.</p>

<blockquote>
  SELECT datetime(1092941466, 'unixepoch', 'localtime');
</blockquote>

<p>Compute the current unix timestamp.</p>

<blockquote>
  SELECT strftime('%s','now');
</blockquote>

<p>Compute the number of days since the signing of the US Declaration
of Independence.</p>

<blockquote>
  SELECT julianday('now') - julianday('1776-07-04');
</blockquote>

<p>Compute the number of seconds since a particular moment in 2004:</p>

<blockquote>
  SELECT strftime('%s','now') - strftime('%s','2004-01-01 02:34:56');
</blockquote>

<p>
Compute the date of the first Tuesday in October
for the current year.
</p>

<blockquote>
  SELECT date('now','start of year','+9 months','weekday 2');
</blockquote>

<p>Compute the time since the unix epoch in seconds 
(like strftime('%s','now') except includes fractional part):</p>

<blockquote>
  SELECT (julianday('now') - 2440587.5)*86400.0;
</blockquote>

<h3>Caveats And Bugs</h3>

<p>The computation of local time depends heavily on the whim 
of politicians and is thus difficult to get correct for 
all locales. In this implementation, the standard C library 
function localtime_r() is used to assist in the calculation of 
local time.  The 
localtime_r() C function normally only works for years
between 1970 and 2037. For dates outside this range, SQLite 
attempts to map the year into an equivalent year within 
this range, do the calculation, then map the year back.</p>


<p>These functions only work for dates between 0000-01-01 00:00:00
and 9999-12-31 23:59:59 (julidan day numbers 1721059.5 through 5373484.5).
For dates outside that range, the results of these
functions are undefined.</p>

<p>Non-Vista Windows platforms only support one set of DST rules. 
Vista only supports two. Therefore, on these platforms, 
historical DST calculations will be incorrect. 
For example, in the US, in 2007 the DST rules changed. 
Non-Vista Windows platforms apply the new 2007 DST rules 
to all previous years as well. Vista does somewhat better
getting results correct back to 1986, when the rules were also changed.</p>

<p>All internal computations assume the 
<a href="http://en.wikipedia.org/wiki/Gregorian_calendar">Gregorian calendar</a>
system.  It is also assumed that every
day is exactly 86400 seconds in duration.</p>


