Difference between 2 date in months and function EVEN

[gexxy]

Hi!
I want a difference between 2 dates and I want it in months
But these are differences of ages of children, so if the difference is of few days it has to result 1 month (with EVEN function or similar)
If I don't put 1 or the 2 dates the space has to be empty
I tried with this

=SI.ERROR(REDONDEA.PAR((C5-D5)/31);"")

that in english is IFERROR and EVEN
but:
- there is an other solution to result the exact months of difference between 2 dates? because the months are not always of 31 days as I put
- with this formula if the difference is of few days it result 2 (?!?)...it should result 1 month
- If I don't write one or two of the dates it results 0, but I want empty space in this case

I send an example
Thanks!