I have two dates in my survey; birthday and a survey date.
Survey should finish if the person is less than 18 in survey date. How can I do this?
There is no built-in function that would allow you to check and validate birthdays. However, there is an example of a custom script in the SurveyGizmo documentation (https://script.surveygizmo.com/help/age-verification) that compares the current date to the date entered by the respondent. Once you have studied the script and tried the example survey, you can copy it into a custom script action in your own survey.
You mention the term ‘survey date’. If this is different from the current (system) date, then you will have to modify the script. The example also expects the date to be entered by the respondent in MM/DD/YYYY (i.e. 6/12/1978) format. This is controlled by the a date validation routine placed in the survey question (https://help.surveygizmo.com/help/date).
If this is your first experience with custom scripting, this might be intimidating. But I think that you will find that once you start playing with the script it becomes easier to understand.