A script to add due and start date support to TaskPaper, forked from the script by andyferra. See http://www.hogbaysoftware.com/wiki/StartAndDueDatesV2 for more information.

Parse Start and Due Dates.applescript

(*
Parse Start and Due Dates script
By Ryan Oldford
Based on work by andyferra (https://gist.github.com/andyferra/64842)
change_case taken from http://www.macosxautomation.com/applescript/sbrt/sbrt-06.html
 
This script is used with TaskPaper to provide better start and due date support, as well as support for repeating tasks.
The script searches for @due and @start tags, converts their values to standard date code values,
then adds the appropriate "diff" tag whose value indicates how far away the due or start date is (negative = past, 0 = today).
Any tasks with a @repeat tag that are done are edited to remove the done tag and have their due tags updated.
 
Due/Start dates allowed:
- Any dates in the standard format (YYYY-MM-DD)
- Natural language dates (i.e. June 15), can use abbreviated months (i.e. jun 15) or lower case (i.e. june 15)
- today
- tomorrow
- Any allowable repeat value (see below)
If no date is given, the date value is set to today.
 
Repeating tasks
Any repeating task needs an @repeat tag with a repeat value.
The repeat values are in the format (repeatType:repeatCount).
Repeat type indicates the type of repeat, and can be any of the following:
- day
- week
- month
- year
- any weekday (can be abbreviated, i.e. tues or tue work the same as tuesday)
Repeat count determines how many of the repeat type have to pass before a new repeat is due.
Examples:
- day:1 = every day
- day:3 = every 3rd day
- week:1 = every week
- month:2 = every 2 months 
- sunday:2 = every 2nd Sunday
These are calculated for done tasks based on the due date. If none is given, today will be used (as stated above).
 
Customization
You can use different tag names for the tags by editing the properties below.
*)
 
-- Edit these if you want to use different tags.
property dueTag : "due"
property startTag : "start"
property asapTag : "asap"
property dateTag : {dueTag, startTag, asapTag}
property repeatTag : "repeat"
property diffStartTag : "diffstart"
property diffDueTag : "diffdue"
property diffAsapTag : "diffasap"
property doneTag : "done"
property errorTag : "error"
property removeTags : {diffStartTag, diffDueTag, diffAsapTag}

property numberSet : {"0", "1", "2", "3", "4", "5", "6", "7", "8", "9"}
property monthSet : {January, February, March, April, May, June, July, August, September, October, November, December}

global vToday
set vToday to current date
set time of vToday to 0

global startingASTIDs
set startingASTIDs to AppleScript's text item delimiters

my clearErrors(errorTag)
--Start by converting any due tags with a text description of the date  
--to an actual date  
my convertDate(dueTag)
my convertDate(startTag)

tell front document of application "TaskPaper"
	
	--Next handle repeat tags. A due tag is not required on these.
	repeat with tpRptTag in (every tag of (every entry) whose name is repeatTag)
		tell entry of tpRptTag
			set tagList to (every tag whose name is dueTag)
			set needDue to false
			if tagList is {} then
				--no due tag, create one, and remember to fill in a date below.
				--use today as the start date
				set tpDueTag to make new tag with properties {name:dueTag}
				copy vToday to vDate
				set needDue to true
			else
				--have a due tag, use that as the start date
				set tpDueTag to first item of tagList
				set dateString to (get value of tpDueTag)
				tell me to set vDate to getDateFromText(dateString)
			end if
			if not needDue then
				set tagList to (every tag whose name is doneTag)
			end if
			--if there was no due tag, or the entry is marked done,
			--the calculate and set the date of the due tag.
			if needDue or tagList is not {} then
				--Delete all extra tags
				repeat with delTag in removeTags
					delete (every tag whose name is delTag)
				end repeat
				
				--Create a date from the repeat value
				set rInfo to (get value of tpRptTag)
				if (vDate is not null) then
					tell me to set vDate to getRepeatDate(rInfo, vDate)
				end if
				
				if vDate is not null then
					--fill in the date, make sure it is not marked done
					tell me to set vDateText to getTextFromDate(vDate)
					set value of tpDueTag to vDateText
					delete (every tag whose name is doneTag)
				else
					--flag unknown repeat value with an error tag
					make new tag with properties {name:errorTag, value:"unknown repeat type"}
				end if
			end if
		end tell
	end repeat
	
	--Add/update the dueWhen tag based on the due date of items that are not done
	repeat with tpDueTag in (every tag of (every entry) whose name is dueTag)
		tell entry of tpDueTag
			set tagList to (every tag whose name is doneTag)
			set errTagList to (every tag whose name is errorTag)
			if (tagList is {}) and (errTagList is {}) then
				--Not marked done
				set dateString to (get value of tpDueTag)
				tell me to set vDate to getDateFromText(dateString)
				if vDate is not null then
					set diffDays to ((round ((vDate - vToday) / days)) as rich text)
					set tagList to (every tag whose name is diffDueTag)
					if (tagList is {}) then
						make new tag with properties {name:diffDueTag, value:diffDays}
					end if
					repeat with thisTag in tagList
						set value of thisTag to diffDays
					end repeat
				else
					--invalid date format
					make new tag with properties {name:"error", value:"invalid date format"}
				end if
			else
				--Marked done, delete all extra tags
				repeat with delTag in removeTags
					delete (every tag whose name is delTag)
				end repeat
			end if
		end tell
	end repeat
	
	--Add/update the startWhen tag based on the start date of items that are not done
	repeat with tpStartTag in (every tag of (every entry) whose name is startTag)
		tell entry of tpStartTag
			set tagList to (every tag whose name is doneTag)
			set errTagList to (every tag whose name is errorTag)
			if (tagList is {}) and (errTagList is {}) then
				--Not marked done
				set dateString to (get value of tpStartTag)
				tell me to set vDate to getDateFromText(dateString)
				if vDate is not null then
					set diffDays to ((round ((vDate - vToday) / days)) as rich text)
					set tagList to (every tag whose name is diffStartTag)
					if (tagList is {}) then
						make new tag with properties {name:diffStartTag, value:diffDays}
					end if
					repeat with thisTag in tagList
						set value of thisTag to diffDays
					end repeat
				else
					--invalid date format
					make new tag with properties {name:"error", value:"invalid date format"}
				end if
			else
				--Marked done, delete all extra tags
				repeat with delTag in removeTags
					delete (every tag whose name is delTag)
				end repeat
			end if
		end tell
	end repeat
	
end tell

on clearErrors(tagLabel)
	tell application "TaskPaper"
		tell front document
			repeat with myTask in entries
				tell myTask
					repeat while (exists tag named tagLabel)
						if (exists tag named tagLabel) then
							delete tag named tagLabel
						end if
					end repeat
				end tell
			end repeat
		end tell
	end tell
end clearErrors

on convertDate(tagLabel)
	tell front document of application "TaskPaper"
		repeat with myTag in (every tag of (every entry) whose name is tagLabel)
			set vDueVal to (value of myTag)
			if vDueVal is missing value then
				set vDueVal to "today"
			end if
			if character 1 of vDueVal is not in numberSet then
				set vDateString to null
				tell me to set vDate to getDateForDueValue(vDueVal)
				if vDate is not null then
					tell me to set vDateString to getTextFromDate(vDate)
				else
					tell entry of myTag
						make new tag with properties {name:errorTag, value:"Unknown value for " & tagLabel & " tag"}
					end tell
				end if
				if vDateString is not null then
					set value of myTag to vDateString
				end if
			end if
		end repeat
	end tell
end convertDate

on getDateFromText(dateText)
	set vDate to null
	set AppleScript's text item delimiters to {"-"}
	if (count of text item of dateText) is 3 then
		set vYear to text item 1 of dateText
		set vMonth to text item 2 of dateText
		set vDay to text item 3 of dateText
		if (vYear > 1000) and (vMonth > 0 and vMonth < 13) and (vDay > 0 and vDay < 32) then
			set vDate to current date
			set year of vDate to (vYear as integer)
			set month of vDate to vMonth as integer
			set day of vDate to vDay as integer
			set time of vDate to 0
		end if
	end if
	return vDate
end getDateFromText

on getTextFromDate(vDate)
	set dText to ((year of vDate) as text) & "-"
	set dayText to (month of vDate as number) as text
	if length of dayText is 1 then
		set dayText to "0" & dayText
	end if
	set dText to dText & dayText & "-"
	set dayText to (day of vDate as number) as text
	if length of dayText is 1 then
		set dayText to "0" & dayText
	end if
	return dText & dayText
end getTextFromDate

on getDateForDueValue(dueValue)
	set vDate to null
	if character 1 of dueValue is in numberSet then
		set vDate to getDateFromText(dueValue)
	else
		set AppleScript's text item delimiters to {" "}
		if (count of text items in dueValue) = 2 then
			-- check for months, do natural language date processing
			copy vToday to vDate
			set firstWord to (first text item of dueValue)
			set secondWord to (second text item of dueValue)
			if (whichMonth(firstWord) is not null) and ((first character of secondWord) is in numberSet) then
				set month of vDate to whichMonth(firstWord)
				set day of vDate to (secondWord as integer)
				set year of vDate to (year of vToday)
				repeat while vDate comes before vToday
					set year of vDate to (year of vDate) + 1
				end repeat
			else
				set vDate to null
			end if
		else if (count of text items in dueValue) = 1 then
			--Special cases for due date not handled in getRepeatDate
			if "today" is dueValue then
				copy vToday to vDate
			else if "tomorrow" starts with dueValue then
				copy vToday to vDate
				set vDate to vDate + 1 * days
			else
				set vDate to getRepeatDate(dueValue, vToday)
			end if
		end if
	end if
	set AppleScript's text item delimiters to startingASTIDs
	return vDate
end getDateForDueValue

on getRepeatDate(repeatDesc, fromDate)
	copy fromDate to vDate
	
	set AppleScript's text item delimiters to {":"}
	set repeatType to text item 1 of repeatDesc
	if (count of text items in repeatDesc) > 1 then
		set repeatCount to text item 2 of repeatDesc
	else
		set repeatCount to 1
	end if
	set AppleScript's text item delimiters to startingASTIDs
	
	if repeatType is "day" or repeatType is "week" then
		if repeatType is "day" then
			set vInterval to days
		else
			set vInterval to weeks
		end if
		set vDate to vDate + repeatCount * vInterval
		repeat while vDate comes before vToday
			set vDate to vDate + repeatCount * vInterval
		end repeat
	else if repeatType is "month" then
		set month of vDate to (month of vDate) + repeatCount
		repeat while vDate comes before vToday
			set month of vDate to (month of vDate) + repeatCount
		end repeat
	else if repeatType is "year" then
		set year of vDate to (year of vDate) + repeatCount
		log "vDate is " & vDate
		repeat while vDate comes before vToday
			set year of vDate to (year of vDate) + repeatCount
		end repeat
	else
		set rDay to 0
		if "sunday" starts with repeatType then
			set rDay to Sunday
		else if "monday" starts with repeatType then
			set rDay to Monday
		else if "tuesday" starts with repeatType then
			set rDay to Tuesday
		else if "wednesday" starts with repeatType then
			set rDay to Wednesday
		else if "thursday" starts with repeatType then
			set rDay to Thursday
		else if "friday" starts with repeatType then
			set rDay to Friday
		else if "saturday" starts with repeatType then
			set rDay to Saturday
		end if
		if rDay > 0 then
			--Handle case where vDate is not currect day of week.
			set vOffset to rDay - (weekday of vDate)
			if vOffset is not 0 then
				set vDate to vDate + vOffset * days
				if vOffset > 0 then
					--If we move forward, count that as 1
					set repeatCount to repeatCount - 1
				end if
			end if
			
			--Find next date after vDate
			set vOffset to 7 * repeatCount
			set vDate to vDate + vOffset * days
			repeat while vDate comes before vToday
				set vDate to vDate + vOffset * days
			end repeat
		else
			--Unknown type, return nothing
			set vDate to null
		end if
	end if
	return vDate
end getRepeatDate

on whichMonth(theText)
	set theText to change_case(theText, 0)
	if length of theText is less than 3 then return null
	set theMonth to null
	repeat with i from 1 to 12
		set aMonth to item i of monthSet
		if (aMonth as text) starts with theText then
			copy aMonth to theMonth
		end if
	end repeat
	return theMonth
end whichMonth

on change_case(this_text, this_case)
	if this_case is 0 then
		set the comparison_string to "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
		set the source_string to "abcdefghijklmnopqrstuvwxyz"
	else
		set the comparison_string to "abcdefghijklmnopqrstuvwxyz"
		set the source_string to "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
	end if
	set the new_text to ""
	repeat with this_char in this_text
		set x to the offset of this_char in the comparison_string
		if x is not 0 then
			set the new_text to (the new_text & character x of the source_string) as string
		else
			set the new_text to (the new_text & this_char) as string
		end if
	end repeat
	return the new_text
end change_case

It would be nice to kepp the @today, @overdue and @upcoming tags. A lot of themes use these tags to color items.

Hi Ryan, I'm researching recurring start dates in TaskPaper. Seems like there's currently no script to do it. Is that right? I see on line 85 you have a note about refreshing a start date, but I can't get this work when I fire the script.

Thanks,
Jack