Calculate Age in Months

I need to be able to calculate age in months, and round whatever remainder to the middle of the month (.5). I am calculating z-scores and percentiles for in a childhood obesity project. The DoB and Date of measurements must be converted to read anything from 142.0 to 142.9 as 142.5. This value will be used to look up the the appropriate LMS (Box Cox, Median, and Standard Deviation) in another excel table. I have to be able to compute age in months so 142.5, 143.5, 144.5, 144.5 etc. It is just not rounding off to the nearest half month. Can anyone help?