<!-- Copyright 2008 Orbitz WorldWide

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

   http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License. -->
<h3><a>AT-STYLE TIME SPECIFICATION</a></h3>

<p>Graphite uses an at-style time specification similar to that used by RRDTool. The specification is called ``at-style'' after the Unix command <code>at(1)</code> that has moderately complex ways to specify time to run your job at a certain date and time. The at-style specification consists of two parts: the <strong>TIME REFERENCE</strong> specification and the <strong>TIME OFFSET</strong> specification.</p>
<h3><a>TIME REFERENCE SPECIFICATION</a></h3>
<p>The time reference specification is used, well, to establish a reference moment in time (to which the time offset is then applied to). When present, it should come first, when omitted, it defaults to <strong>now</strong>. On its own part, time reference consists of a <em>time-of-day</em> reference (which should come first, if present) and a <em>day</em> reference.</p>

<p>The <em>time-of-day</em> can be specified as <strong>HH:MM</strong>, <strong>HH.MM</strong>, or just <strong>HH</strong>. You can suffix it with <strong>am</strong> or <strong>pm</strong> or use 24-hours clock. Some special times of day are understood as well, including <strong>midnight</strong> (00:00), <strong>noon</strong> (12:00) and British <strong>teatime</strong> (16:00).</p>

<p>The <em>day</em> can be specified as <em>month-name</em> <em>day-of-the-month</em> and optional a 2- or 4-digit <em>year</em> number (e.g. March 8 1999). Alternatively, you can use <em>day-of-week-name</em> (e.g. Monday), or one of the words: <strong>yesterday</strong>, <strong>today</strong>, <strong>tomorrow</strong>. You can also specify the <em>day</em> as a full date in several numerical formats, including <strong>MM/DD/[YY]YY</strong>, <strong>DD.MM.[YY]YY</strong>, or <strong>YYYYMMDD</strong>.</p>

<p><em>NOTE1</em>: this is different from the original <code>at(1)</code> behavior, where a single-number date is interpreted as MMDD[YY]YY.</p>
<p><em>NOTE2</em>: if you specify the <em>day</em> in this way, the <em>time-of-day</em> is REQUIRED as well.</p>
<p>Finally, you can use the words <strong>now</strong>, <strong>start</strong>, or <strong>end</strong> as your time reference. <strong>Now</strong> refers to the current moment (and is also the default time reference). <strong>Start</strong> (<strong>end</strong>) can be used to specify a time relative to the start (end) time for those tools that use these categories.</p>

<p>Month and day of the week names can be used in their naturally abbreviated form (e.g., Dec for December, Sun for Sunday, etc.). The words <strong>now</strong>, <strong>start</strong>, <strong>end</strong> can be abbreviated as <strong>n</strong>, <strong>s</strong>, <strong>e</strong>.</p>
<h3><a>TIME OFFSET SPECIFICATION</a></h3>

<p>The time offset specification is used to add/subtract certain time intervals to/from the time reference moment. It consists of a <em>sign</em> (<strong>+</strong> or <strong>-</strong>) and an <em>amount</em>. The following time units can be used to specify the <em>amount</em>: <strong>years</strong>, <strong>months</strong>, <strong>weeks</strong>, <strong>days</strong>, <strong>hours</strong>, <strong>minutes</strong>, or <strong>seconds</strong>. These units can be used in singular or plural form, and abbreviated naturally or to a single letter (e.g. +3days, -1wk, -3y). Several time units can be combined (e.g., -5mon1w2d) or concatenated (e.g., -5h45min = -5h-45min = -6h+15min = -7h+1h30m-15min, etc.)</p>

<p><em>NOTE3</em>: If you specify time offset in days, weeks, months, or years, you will end with the time offset that may vary depending on your time reference, because all those time units have no single well defined time interval value (1 year contains either 365 or 366 days, 1 month is 28 to 31 days long, and even 1 day may be not equal to 24 hours twice a year, when DST-related clock adjustments take place). To cope with this, when you use days, weeks, months, or years as your time offset units your time reference date is adjusted accordingly without too much further effort to ensure anything about it (in the hope that <code>mktime(3)</code> will take care of this later). This may lead to some surprising (or even invalid!) results, e.g. 'May 31 -1month' = 'Apr 31' (meaningless) = 'May 1' (after <code>mktime(3)</code> normalization); in the EET timezone '3:30am Mar 29 1999 -1 day' yields '3:30am Mar 28 1999' (Sunday) which is an invalid time/date combination (because of 3am -&gt; 4am DST forward clock adjustment, see the below example).</p>
<p>In contrast, hours, minutes, and seconds are well defined time intervals, and these are guaranteed to always produce time offsets exactly as specified (e.g. for EET timezone, '8:00 Mar 27 1999 +2 days' = '8:00 Mar 29 1999', but since there is 1-hour DST forward clock adjustment that occurs around 3:00 Mar 28 1999, the actual time interval between 8:00 Mar 27 1999 and 8:00 Mar 29 1999 equals 47 hours; on the other hand, '8:00 Mar 27 1999 +48 hours' = '9:00 Mar 29 1999', as expected)</p>
<p><em>NOTE4</em>: The single-letter abbreviation for both <strong>months</strong> and <strong>minutes</strong> is <strong>m</strong>. To disambiguate them, the parser tries to read your mind :) by applying the following two heuristics:</p>

<ol>
<li>If <strong>m</strong> is used in context of (i.e. right after the) years, months, weeks, or days it is assumed to mean <strong>months</strong>, while in the context of hours, minutes, and seconds it means minutes. (e.g., in -1y6m or +3w1m <strong>m</strong> is interpreted as <strong>months</strong>, while in -3h20m or +5s2m <strong>m</strong> the parser decides for <strong>minutes</strong>).</li>

<li>Out of context (i.e. right after the <strong>+</strong> or <strong>-</strong> sign) the meaning of <strong>m</strong> is guessed from the number it directly follows. Currently, if the number's absolute value is below 25 it is assumed that <strong>m</strong> means <strong>months</strong>, otherwise it is treated as <strong>minutes</strong>. (e.g., -25m == -25 minutes, while +24m == +24 months)</li>

</ol>
<p><em>Final NOTES</em>: Time specification is case-insensitive. Whitespace can be inserted freely or omitted altogether. There are, however, cases when whitespace is required (e.g., 'midnight Thu'). In this case you should either quote the whole phrase to prevent it from being taken apart by your shell or use '_' (underscore) or ',' (comma) which also count as whitespace (e.g., midnight_Thu or midnight,Thu).</p>
<h3><a>TIME SPECIFICATION EXAMPLES</a></h3>
<p><em>Oct 12</em> -- October 12 this year</p>
<p><em>-1month</em> or <em>-1m</em> -- current time of day, only a month before (may yield surprises, see NOTE3 above).</p>

<p><em>noon yesterday -3hours</em> -- yesterday morning; can also be specified as <em>9am-1day</em>.</p>
<p><em>23:59 31.12.1999</em> -- 1 minute to the year 2000.</p>
<p><em>12/31/99 11:59pm</em> -- 1 minute to the year 2000 for imperialists.</p>
<p><em>12am 01/01/01</em> -- start of the new millennium</p>

<p><em>end-3weeks</em> or <em>e-3w</em> -- 3 weeks before end time (may be used as start time specification).</p>
<p><em>start+6hours</em> or <em>s+6h</em> -- 6 hours after start time (may be used as end time specification).</p>
<p><em>931225537</em> -- 18:45 July 5th, 1999 (yes, seconds since 1970 are valid as well).</p>

<p><em>19970703 12:45</em> -- 12:45 July 3th, 1997 (my favorite, and its even got an ISO number (8601)).</p>