jeanbernard points to a good solution: indeed the century-month codes were designed exactly for the reason that you can avoid specifying the full date in the case when the day is not known, which is sufficient for many situations, such as measuring the age of a child in months for the z-score functions, and other similar applications.
CenturyMonthCode(month, year) - computes the century month code based on the month and the year.
Note that while you can calculate a century month code conveniently with this function, entry is not as convenient since there is no corresponding question type for it.
Still see if the example section “Calculate child age in months” in “Public example User questions and common patterns” is delivering the desired experience.
Best, Sergiy Radyakn