Shell Script : Calculate Between Two Days

I tested this on LINUX

Use as you please.

# date
Mon May 18 16:16:29 EDT 2015
# date '+%s' -d "now"
1431980193
# date '+%s' -d "May 18 2014"
1400385600
# echo $(( (1431980193 - 1400385600) / 86400 )) 
365