Date Constraints in XLSForm: No Future Dates

To stop future dates in XLSForm, put . <= today() in the constraint column of the date question. The dot is the date being entered and today() is the device's current date, so tomorrow is rejected and today is allowed. For a fixed range, wrap each limit in date('YYYY-MM-DD'): . >= date('2026-01-01') and . <= today(). ODK, KoboToolbox and SurveyCTO all read this syntax. Every example below compiles with pyxform and ODK Validate, the check ODK Central runs when you upload a form.

Download example form (.xlsx)All seven examples in one XLSForm, one group each. Tested with pyxform and ODK Validate; free to reuse under CC BY 4.0.Download

How do I stop future dates in XLSForm?

Compare the answer with today(). Date of birth, date of the last illness and the date a household moved in can never be after today, so each gets the same rule:

type	name	label	required	constraint	constraint_message
date	dob	What is the respondent's date of birth?	yes	. <= today()	The date of birth cannot be in the future.
date	illness_date	When did the child's fever start?	yes	. <= today()	The date cannot be after today.

◀ ▶surveychoicessettings+

Use <=, not <. With . < today() an illness that started this morning is rejected, and the enumerator has no way to enter it. For the opposite case, such as the date of a planned follow-up visit, flip the sign: . >= today() allows today and any later date.

How do I allow only today's date?

Write . = today(). A field team often asks for this for the interview date, so a back-dated interview cannot be entered:

type	name	label	required	constraint	constraint_message
date	interview_date	Date of interview	yes	. = today()	Enter today's date. Interviews cannot be back-dated.

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If the answer must always be today, you may not need a question at all. A today metadata row (type today, name today) records the date with no typing, and the enumerator cannot change it.

How do I limit a date to a fixed range?

Write each fixed limit as date('YYYY-MM-DD'), with a four-digit year, two-digit month and two-digit day. This baseline only accepts household members born from 1 January 1940 up to today:

type	name	label	required	constraint	constraint_message
date	member_dob	What is this person's date of birth?	yes	. >= date('1940-01-01') and . <= today()	Enter a date between 1 January 1940 and today.
date	distribution_date	When did the household receive the last food distribution?	yes	. >= date('2026-07-01') and . <= date('2026-09-30')	The distribution ran from 1 July to 30 September 2026.

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Two details make or break this rule:

• Wrap the date in date(). A bare '2026-07-01' is text. pyxform and ODK Validate accept it, but ODK Collect compares a date with text and the rule never passes.
• Use the ISO order. date('01/07/2026') also uploads without an error, but Collect cannot read it. A KoboToolbox forum user found the same rule working in Enketo and failing in Collect for exactly this reason.

How do I allow only the last 30 days?

Turn both dates into day numbers with decimal-date-time(), then subtract the number of days. decimal-date-time() counts days since 1 January 1970, so 30 days ago is decimal-date-time(today()) - 30. This recall question for a health facility visit accepts the last 30 days, including today:

type	name	label	required	constraint	constraint_message
date	visit_date	When did the child last visit a health facility?	yes	decimal-date-time(.) >= decimal-date-time(today()) - 30 and . <= today()	The visit must be in the last 30 days.
date	rain_date	When did it last rain enough to plant?	yes	decimal-date-time(.) >= decimal-date-time(today()) - 365 and . <= today()	Enter a date in the last 12 months.

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This is the form an ODK developer suggests on the forum when a rule like . >= today() - 180 does not behave as expected. Keep the . <= today() part: without it, any future date also passes, because a future date is always more recent than 30 days ago.

How do I check one date against another?

Reference the earlier question with ${name}. A treatment end date must be on or after its start date and not in the future:

type	name	label	required	constraint	constraint_message
date	treat_start	When did the treatment start?	yes	. <= today()	The start date cannot be in the future.
date	treat_end	When did the treatment end?	yes	. >= ${treat_start} and . <= today()	The end date must be between the start date and today.
date	follow_up	When is the follow-up visit planned?	yes	decimal-date-time(.) >= decimal-date-time(${treat_start}) + 7 and decimal-date-time(.) <= decimal-date-time(${treat_start}) + 90	Plan the follow-up 7 to 90 days after the treatment started.

◀ ▶surveychoicessettings+

The follow_up rule adds days to another answer the same way the last-30-days rule subtracts them from today. The earlier question must come first in the form: a constraint that names a later, still empty question compares with nothing. The general pattern of comparing two answers is in the XLSForm constraint examples.

How do I require a minimum age from the date of birth?

Compare the birth date with the date exactly 18 years ago. Writing both as YYYYMMDD numbers with format-date() gives an exact birthday check, with no rounding of 365.25 days:

type	name	label	required	constraint	constraint_message
date	resp_dob	What is your date of birth?	yes	format-date(., '%Y%m%d') <= concat(format-date(today(), '%Y') - 18, format-date(today(), '%m%d'))	You must be 18 or older to take part in this survey.

◀ ▶surveychoicessettings+

On 1 November 2026 the right side is 20081101, so anyone born on or before 1 November 2008 passes. Change the 18 for another threshold, for example 15 for women of reproductive age. To show the age itself, see calculating age from date of birth.

Does the same rule work for dateTime questions?

Yes, with now() in place of today(). A dateTime question records the time too, so . <= now() blocks a time later than the current moment:

type	name	label	required	constraint	constraint_message
dateTime	birth_time	When was the baby born (date and time)?	yes	. <= now() and . >= date('2026-01-01')	Enter a time in 2026 up to now.

◀ ▶surveychoicessettings+

today() has no time part, so comparing a dateTime answer with today() treats today as midnight and rejects a birth earlier this morning. Use now() for dateTime and today() for date. To show a date as DD-MM-YYYY in a note, use format-date(), covered in formatting dates in XLSForm.

Common mistakes and how to fix them

We ran each of these through pyxform and ODK Validate. The messages are what you will see in ODK Central, KoboToolbox or the XLSForm validator:

You wrote	Write instead	Error you will see
. => date('2021-01-01')	. >= date('2021-01-01')	contains invalid constraint expression [. => date('2021-01-01')] Bad node
. <= ${today} (no question named today)	. <= today()	Could not find the name 'today'.
. >= ${start_date} where the question is treat_start	The real name: . >= ${treat_start}	Could not find the name 'start_date'.
. <= today	. <= today()	No error, but today without brackets is read as a field path, not the function, so every date is rejected
. >= '2021-01-01'	. >= date('2021-01-01')	No error, but ODK Collect compares the date with text and rejects every date
. >= date('01/01/2021')	. >= date('2021-01-01')	No error at upload; Collect cannot read the date
int((today() - .) div 365.25) <= 28 for "last 4 weeks"	decimal-date-time(.) >= decimal-date-time(today()) - 28	No error, but it counts years, not days, so almost any date passes

The last four rows pass every check and fail in the field. Test each limit in the form preview before you deploy: one date just inside and one just outside. For other compile messages, see common XLSForm errors.

Does this work the same in ODK, KoboToolbox and SurveyCTO?

The syntax is shared. Where you type it differs:

Platform	Where the rule goes	Notes
ODK Collect and Central	constraint and constraint_message columns	today(), now(), date(), decimal-date-time() and format-date() as above
KoboToolbox (KoboCollect and Enketo)	Same XLSForm columns; in the Formbuilder, the question's Validation Criteria	Same functions. Test date rules in KoboCollect as well as Enketo: forum threads report rules that behave differently between the two
SurveyCTO	constraint and constraint message columns	today(), now(), date() and format-date() work the same. Check SurveyCTO's function list before copying a day-window rule

In all three, a constraint only runs when the question is shown and answered. If skip logic hides a date question, its rule never fires.

Why date rules are worth the effort

Dates feed the numbers people report: under-5 status, recall windows, time from symptom to treatment. A year typed as 2062 instead of 2026, or an illness dated next week, cannot be fixed after the household is gone. A date rule catches it while the respondent is still there to give the right answer, which is cheaper than any amount of cleaning later.

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