How to count days since a specific date until today using Bash shell ?

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