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

Objective

The objective is to use bash shell to count days since a specific date until now ( today ).

Difficulty

EASY

Conventions

• # – requires given linux commands to be executed with root privileges either directly as a root user or by use of sudo command
• $ – requires given linux commands to be executed as a regular non-privileged user

Instructions

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 lets 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 1.1.2017 and 31.12.2017 dates:

$ echo $((($(date +%s --date "2017-12-31")-$(date +%s --date "2017-1-1"))/(3600*24))) days
364 days