How to Subtract a Year from a Date Stored in a Variable in Shell Script

How to subtract a year from a date stored in a variable in shell script?

Played around with it. This seems to work:

as_of_dt='2016-01-01'
as_of_dt_prev_year=$(date --date="${as_of_dt} -1 year" +'%Y-%m-%d')
echo $as_of_dt_prev_year

Note the double quotes that are needed for variable substitution to work.

How to subtract or add date from a variable?

With GNU date it can be done quite easily with its -d switch.

x=20170402
date -d "$x -1 days" "+%Y%m%d"
20170401

and for 2 days

date -d "$x - 2 days" "+%Y%m%d"
20170331

subtract days from a date in bash

You are specifying the date incorrectly. Instead, say:

date --date="${dataset_date} -${date_diff} day" +%Y-%m-%d

If you need to store it in a variable, use $(...):

p_dataset_date=$(date --date="${dataset_date} -${date_diff} day" +%Y-%m-%d)

Date conversion and subtraction in shell script

A quick solution :

datetime=20180123
date -d "$datetime - $date_diff days" +%Y-%m-%d
# -----------------------------^^^^^^

returns

2018-01-21

Note that I had to eliminate the time portion of your time-stamp and used -2 days for the subtraction.

If you need the time portion, I would recommend saving that to a separate variable then appending that value back onto your output. But as you've changed your output format, I guess the time isn't that important?

IHTH

How to add or subtract from date with format DDMMYYYY in bash?

From man date

The --date=STRING is a mostly free format human readable date string such as "Sun, 29 Feb 2004 16:21:42 -0800" or "2004-02-29 16:21:42" or even "next Thursday". [...] The date string format is more complex than is easily documented here but is fully described in the info documentation.

tl;dr: Seems like you cannot specify the format of the input. Use one of the known formats, for instance

date -d '2019-12-13 -5 day' +%d%m%Y

To automatically convert a date from DDMMYYYY format to YYYY-MM-DD you can use sed ...

date -d "$(sed -E 's/(..)(..)(.*)/\3-\2-\1/' <<< 13122019) -5 day" +%d%m%Y

... or bash ...

d=13122019
date -d "${d:4}-${d:2:2}-${d:0:2} -5 day" +%d%m%Y

Today's date, minus X days in shell script

For GNU date:

date_222days_before_TodayYear=$(date --date="222 days ago" +"%Y")
date_222days_before_TodayMonth=$(date --date="222 days ago" +"%m")
date_222days_before_TodayDay=$(date --date="222 days ago" +"%d")

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For BSD date::

If you are using OS X or FreeBSD, use the following instead because BSD date is different from GNU date:

date_222days_before_TodayYear=$(date -j -v-222d +"%Y")
date_222days_before_TodayMonth=$(date -j -v-222d +"%m")
date_222days_before_TodayDay=$(date -j -v-222d +"%d")

Source: BSD date manual page

Note:

In bash and many other languages, you cannot start a variable name with a numerical character, so I prefixed them with date_ for you.

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Second Update: New requirement - Using 222 Working days instead of 222 Regular days:

(Assumption: Not considering statutory holidays, because that just gets far beyond the scope of what I can help you with in a shell script:)

Consider 222 working days:

• 5 working days per week, that is floor(222/5) == 44 weeks
• 44 weeks * 7 days per week == 308 days
• Extra days leftover: 222 % 5 == 2
• Therefore 222 working days == 310 regular days

But, there is a catch! If the number of regular days is 308 or some multiple of 7, then we would have been fine, because any multiple of 7-days ago from a working day is still a working day. So we need to consider whether today is a Monday or a Tuesday:

• If today is a Monday, we'd get Saturday where we wanted Thursday
• If today is a Tuesday, we'd get Sunday where we wanted Friday

So you see we need an additional offset of 2 more days if today is either Monday or Tuesday; so let's find that out first before we proceed:

#!/bin/bash

# Use 310 days as offset instead of 222
offset=310
# Find locale's abbreviated weekday name (e.g., Sun)
today=$(date -j +"%a")
# Check for Mon/Tue
if [[ "$today" == "Mon" ]] || [[ "$today" == "Tue" ]]; then
     offset=$((offset+2))
fi

date_222_working_days_before_TodayYear=$(date -j -v-${offset}d +"%Y")
date_222_working_days_before_TodayMonth=$(date -j -v-${offset}d +"%m")
date_222_working_days_before_TodayDay=$(date -j -v-${offset}d +"%d")

And that should do it =)

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