The below scripts shows output number of days between  two days. there are different format are below,

%s     seconds 
-d     option to convert the input string into date format as shown below.

Seconds :

 echo $(( ( $(date -ud '2015-10-01 10:10:10' +'%s') - $(date -ud '2015-09-10 10:10:10' +'%s') ) )) seconds

1814400 seconds

Minutes :

echo $(( ( $(date -ud '2015-10-01 10:10:10' +'%s') - $(date -ud '2015-09-10 10:10:10' +'%s') )/60 )) minutes

30240 minutes

Hours :

echo $(( ( $(date -ud '2015-10-01 10:10:10' +'%s') - $(date -ud '2015-09-10 10:10:10' +'%s') )/60/60 )) hours

504 hours

Days:

Calculate days in different format:

echo $(($(($(date -d "2010-06-01" "+%s") - $(date -d "2010-05-15" "+%s"))) / 86400))

17

echo $(($(($(date -d "Oct 10" "+%s") - $(date -d "sep 2" "+%s"))) / 86400))

38