Hi,
though I do not see any necessity not to use a multiple numerical input or an array(numbers) with one row and two columns here you find two solutions.
The first with dropdowns.
In my opinion the number of month is limited to a few (otherwise the number of days is quite senseless).
So a dropdown is the best way to avoid silly inputs like 0 months, 45 days.
forums.limesurvey.org/forum/can-i-do-thi...down-box-within-text