Calculate difference in months between two dates in Unix?

Question

I have two variables in the format 'YYYYMM'

DATE1=201712
DATE2=201801

How do I calculate the difference between these two date variables in months in unix?

Answer 1

How about:

#!/bin/bash

DATE1=201712
DATE2=201801

y1=${DATE1:0:4}
m1=${DATE1:4:2}

y2=${DATE2:0:4}
m2=${DATE2:4:2}

diff=$(( ($y2 - $y1) * 12 + (10#$m2 - 10#$m1) ))
echo $diff