IncrementDateValue

# Uses a yyyymmdd(.hh) date format.

on add_leading_zeros(i, desired_length)
	set i to i as number
	set i_length to (length of (i as string))
	if i_length ≥ desired_length then return i as string
	text -desired_length thru -1 of ("00000000000000000000000000000000" & i)
end add_leading_zeros

on pretty_date(_date)
	set _year to year of _date
	set _month to my add_leading_zeros((month of _date), 2)
	set _day to my add_leading_zeros((day of _date), 2)
	_year & _month & _day as string
end pretty_date

on parse_date(s)
	set _year to (text 1 thru 4 of s)
	set _month to (text 5 thru 6 of s)
	set _day to (text 7 thru 8 of s)
	date (_month & "/" & _day & "/" & _year)
end parse_date

on valid_date(s)
	if s = missing value then return false
	try
		parse_date(s)
	on error
		return false
	end try
	true
end valid_date

tell application "TaskPaper"
	tell selected entry
		if (exists tag named "date") and ¬
			not my valid_date(value of tag named "date") then
			delete tag named "date"
		end if
		if (exists tag named "date") then
			set _date to my parse_date(value of tag named "date")
			set _date to _date + (1 * days)
			set value of tag named "date" to my pretty_date(_date)
		else
			make tag with properties {name:"date", value:my pretty_date(current date)}
		end if
	end tell
end tell

(*
Copyright (c) 2010, Nate Bunnyfield
All rights reserved.

Redistribution and use in source and binary forms, with or without
modification, are permitted provided that the following conditions are met:

Redistributions of source code must retain the above copyright notice, this
list of conditions and the following disclaimer. Redistributions in binary
form must reproduce the above copyright notice, this list of conditions and
the following disclaimer in the documentation and/or other materials provided
with the distribution. Neither the name of the PEEK PEEK POKE nor the names of
its contributors may be used to endorse or promote products derived from this
software without specific prior written permission.

THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
*)