/** \file parsedate.h
\brief date parsing functions

This is a set a functions for parsing date strings in various formats.
It's mostly tailored for parsing user given data, although originally,
it was developed to parse the date strings found in usenet messages.

The given date will be parsed relative to the specified time, and using
a predefined set of time/date formats.

\par Valid Input Strings

The internal formats allow parsedate() to understand a wide range of
input strings. The format list is ought to be compiled from the Date: 
line of 80.000 usenet messages.

But since this function is also used in end-user applications like the
Tracker's find panel, it's helpful to know what this function accepts
and what not.

Here are some examples of input strings that parsedate() will be able
to convert along with some notes:
	- "last friday", "this wednesday", "next July"
		"last", "next", and "this" refer to the week or year (depending
		on the context). So "last friday" means last week's friday.
		"This wednesday" is referring to this week's wednesday, no matter
		if it has already passed or not.
		"Next July" refers to next year's July. All of these dates are
		parsed relative to the specified time (usually "now"), and will
		be set to the first moment of that time span: "next monday" is
		monday, 0:00:00, midnight.
	- "now" just returns the time all calculations are relative to.
	- "next 5 minutes", "5 minutes", "+5 mins" all mean the same thing,
		that is, current time plus exactly 5 minutes.
	- "5 weeks" means in 5 weeks from now on.
	- "8/5/2003", "5.8.2003", "2003-08-05" are all referring to August
		5th, 2003, again at 0:00 midnight.
	- "Thursday 3:00" means this week's thursday, at 3 o'clock.

\anchor parsedateFormats 
\par Format Specifier

While the get_dateformats() function allow you to retrieve the built-in
formats, you can also define your own and use set_dateformats() to let
parsedate() use them in all subsequent calls.

The following is a list valid format specifiers and their meanings.

	- \b a/A weekday (Sunday, Monday, ...)
	- \b d day of month (1-31)
	- \b b/B month name (January, February, ...)
	- \b month (1-12)
	- \b y/Y year
	- \b H/I hours (1-24)
	- \b M minutes (0-60)
	- \b S seconds (0-60)
	- \b p meridian (am/pm)
	- \b z/Z time zone (i.e. GMT)
	- \b T time unit, like "last friday", "next 5 minutes", "-15 hours", etc.
	- \b - dash or slash

Any of ",.:" is allowed and will be expected in the input string as is.
You can enclose a \b single field with "[]" to mark it as being optional.
A blank stands for white space. No other character is allowed.
An invalid format string won't do any harm, but of course, no input string
will ever match that format.

For example, "H:M [p]" will match against "21:33", "4:12 am", but not "30:30 pm" 
(hours out of range), "15:16 GMT" (this time zone is certainly not a valid 
meridian specifier), or "4:66" (minutes out of range).

\par Note:
At the time of this writing, the parsedate() functions are not localized and
will only recognize English time specifications following the examples above.

*/

/** \def PARSEDATE_RELATIVE_TIME
\brief relative time

The time value was computed relative to the specified time.
*/

/** \def PARSEDATE_DAY_RELATIVE_TIME
\brief day relative time

The time value was computed relative to the specified time, and it would vary with
every day passed in the specified time.
*/

/** \def PARSEDATE_MINUTE_RELATIVE_TIME
\brief minute relative time

The time value was computed relative to the specified time, and it would vary with
every minute passed in the specified time.
*/

/** \def PARSEDATE_INVALID_DATE
\brief invalid date string

This flag will be set if the specified date string could not be parsed correctly.
For example, this may happen if there are some unknown words in that string.
*/

/** \fn time_t parsedate(const char *dateString, time_t relativeTo)
\brief Parses <span class="var">dateString</span> relative to <span class="var">relativeTo</span>

Parses the given <span class="var">dateString</span> relative to the time 
specified by <span class="var">relativeTo</span> using the internal formats 
table.

\param dateString the date that should be parsed, i.e. "next thursday"
\param relativeTo all relative dates will be relative to this time, if -1 is passed, the current time will be used
\return the parsed time value or -1 if the <span class="var">dateString</span> 
could not be parsed.
*/

/** \fn time_t parsedate_etc(const char *dateString, time_t relativeTo, int *_storedFlags) 
\brief Parses <span class="var">dateString</span> relative to <span class="var">relativeTo</span>

This does basically the same as parsedate(), but will set the following 
flags in <span class="var">_storedFlags</span>:
\htmlonly
	<table border=1>
<!-- ToDo: this certainly is a hack -->
		<tr><th bgcolor="#eeeeee">Constant</th><th bgcolor="#eeeeee">Meaning</th></tr>
		<tr><td class="mdname1">PARSEDATE_RELATIVE_TIME</td>
			<td>\endhtmlonly \copydoc PARSEDATE_RELATIVE_TIME \htmlonly
			</td></tr>
		<tr><td class="mdname1">PARSEDATE_DAY_RELATIVE_TIME</td>
			<td>\endhtmlonly \copydoc PARSEDATE_DAY_RELATIVE_TIME \htmlonly
			</td></tr>
		<tr><td class="mdname1">PARSEDATE_MINUTE_RELATIVE_TIME</td>
			<td>\endhtmlonly \copydoc PARSEDATE_MINUTE_RELATIVE_TIME \htmlonly
			</td></tr>
		<tr><td class="mdname1">PARSEDATE_INVALID_DATE</td>
			<td>
				\endhtmlonly \copydoc PARSEDATE_INVALID_DATE \htmlonly
				This flag will only be set if the function returns -1.
			</td></tr>
	</table>
\endhtmlonly
*/

/** \fn void set_dateformats(const char *formatTable[])
\brief sets the internal format table for parsedate()

This function let you set the format table which is used by parsedate().
When <span class="var">formatTable</span> is NULL, the standard built-in format table will be set again.
\param formatTable the NULL terminated formats list. This list must stay
	valid when using parsedate() - it is not copied, but directly used.
\see 
	\ref parsedateFormats Format!

*/

/** \fn const char **get_dateformats(void)
\brief returns the internal format table currently used by parsedate()

Returns the internal format table currently used by parsedate() - this is
either a pointer to the built-in one, or one that you have previously
set using set_dateformats().

\see
	\ref set_dateformats()
*/