Use this template to add a month to a date, e.g. if the next visit has to be within a month after the first visit. Replace {date} with your own variable name.

var date = moment('{date}', 'DD-MM-YYYY');
var newDate = date.add(1, 'months');
newDate.format('DD-MM-YYYY');

Test this template in the calculation helper.

If you want to use different time units, you can change the second line of the code. For example, if you want to add 2 years to your date instead of 1 month, you can use:

var newDate = date.add(2, 'years');

Looking to subtract from a date? See the article Subtract a month from a date how you can subtract a month from a date.