The easiest way to perform days count since a specifics day is to first get a number of seconds
since epoch time ( 1970-01-01 ) for both dates. As an example let’s count number of days
since 28.12.1999 until today 8.1.2018. Consider a following example:
$ echo $((($(date +%s)-$(date +%s –date “1999-12-28”))/(3600*24))) days
6586 days
Let’s add little bit of readability to the above command by using variables. First, we get
seconds since epoch time ( 1970-01-01 ) until now:
$ now=$(date +%s)
$ echo $now
1515370378
Next we do the same for the 28.12.1999 date:
past=$(date +%s –date “1999-12-28”)
$ echo $past
946299600
Next, calculate the difference:
$ difference=$(($now-$past))
$ echo $difference
569070778
Lastly, convert the difference in seconds to days:
$ echo $(($difference/(3600*24)))
6586
All done. The same principle can be used to calculate days between any specific days.
For example let’s count days between 1.1.2016 and 31.12.2016 dates:
$ echo $((($(date +%s –date “2016-12-31”)-$(date +%s –date “2016-1-1”))/(3600*24))) days
365 days