I am trying to compute inter-pregnancy intervals (in months) and have the variables to compute this interval between two dates. However, the "day of birth" variable is missing. Dataset only has (mm) and years (yy). My goal: To calculate the pregnancy interval between most recent birth and previous live birth in months.

The numeric variables are listed below;
Most recent birth
1. Month of most recent birth (mm_dob)
2. Year of most recent birth (yy4_dob)
Previous live birth
3. Month of last live birth (mm_llb)
4. Year of last live birth (yy4_llb)

I will be most grateful for your help.