You can just use the -d
switch and provide a date to be calculated
date
Sun Sep 23 08:19:56 BST 2012
NEW_expration_DATE=$(date -d "+10 days")
echo $NEW_expration_DATE
Wed Oct 3 08:12:33 BST 2012
-d, --date=STRING
display time described by STRING, not ‘now’
This is quite a powerful tool as you can do things like
date -d "Sun Sep 11 07:59:16 IST 2012+10 days"
Fri Sep 21 03:29:16 BST 2012
or
TZ=IST date -d "Sun Sep 11 07:59:16 IST 2012+10 days"
Fri Sep 21 07:59:16 IST 2012
or
prog_end_date=`date '+%C%y%m%d' -d "$end_date+10 days"`
So if $end_date=20131001 then $prog_end_date=20131011
That last example should have clarified things for you: timezones.
$ TZ=UTC date -d "2019-01-19T05:00:00Z - 2 hours" +%Y-%m-%d_%H:%M:%S
2019-01-19_03:00:00
$ TZ=Asia/Colombo date -d "2019-01-19T05:00:00Z - 2 hours" +%Y-%m-%d_%H:%M:%S
2019-01-19_08:30:00
As the output clearly varies by the timezone, I'd suspect some non-obvious default taken for a time string without a timezone specified. Testing a couple of values, it seems to be UTC-05:00, though I'm not sure what that is.
$ TZ=UTC date -d "2019-01-19T05:00:00 - 2 hours" +%Y-%m-%d_%H:%M:%S%Z
2019-01-19_08:00:00UTC
$ TZ=UTC date -d "2019-01-19T05:00:00Z - 2 hours" +%Y-%m-%d_%H:%M:%S%Z
2019-01-19_03:00:00UTC
$ TZ=UTC date -d "2019-01-19T05:00:00" +%Y-%m-%d_%H:%M:%S%Z
2019-01-19_05:00:00UTC
It's only used when performing date arithmetic.
It seems the issue here is that - 2 hours
is not taken as arithmetic, but as a timezone specifier:
# TZ=UTC date -d "2019-01-19T05:00:00 - 2 hours" +%Y-%m-%d_%H:%M:%S%Z --debug
date: parsed datetime part: (Y-M-D) 2019-01-19 05:00:00 UTC-02
date: parsed relative part: +1 hour(s)
date: input timezone: parsed date/time string (-02)
date: using specified time as starting value: '05:00:00'
date: starting date/time: '(Y-M-D) 2019-01-19 05:00:00 TZ=-02'
date: '(Y-M-D) 2019-01-19 05:00:00 TZ=-02' = 1547881200 epoch-seconds
date: after time adjustment (+1 hours, +0 minutes, +0 seconds, +0 ns),
date: new time = 1547884800 epoch-seconds
date: timezone: TZ="UTC" environment value
date: final: 1547884800.000000000 (epoch-seconds)
date: final: (Y-M-D) 2019-01-19 08:00:00 (UTC)
date: final: (Y-M-D) 2019-01-19 08:00:00 (UTC+00)
2019-01-19_08:00:00UTC
So, not only is no arithmetic being done, there seems to be a daylight savings 1 hour adjustment on the time, leading to a somewhat nonsensical time for us.
This also holds for addition:
# TZ=UTC date -d "2019-01-19T05:00:00 + 5:30 hours" +%Y-%m-%d_%H:%M:%S%Z --debug
date: parsed datetime part: (Y-M-D) 2019-01-19 05:00:00 UTC+05:30
date: parsed relative part: +1 hour(s)
date: input timezone: parsed date/time string (+05:30)
date: using specified time as starting value: '05:00:00'
date: starting date/time: '(Y-M-D) 2019-01-19 05:00:00 TZ=+05:30'
date: '(Y-M-D) 2019-01-19 05:00:00 TZ=+05:30' = 1547854200 epoch-seconds
date: after time adjustment (+1 hours, +0 minutes, +0 seconds, +0 ns),
date: new time = 1547857800 epoch-seconds
date: timezone: TZ="UTC" environment value
date: final: 1547857800.000000000 (epoch-seconds)
date: final: (Y-M-D) 2019-01-19 00:30:00 (UTC)
date: final: (Y-M-D) 2019-01-19 00:30:00 (UTC+00)
2019-01-19_00:30:00UTC
Debugging a bit more, the parsing seems to be: 2019-01-19T05:00:00 - 2
(-2
being the timezone), and hours
(= 1 hour), with an implied addition. It becomes easier to see if you use minutes instead:
# TZ=UTC date -d "2019-01-19T05:00:00 - 2 minutes" +%Y-%m-%d_%H:%M:%S%Z --debug
date: parsed datetime part: (Y-M-D) 2019-01-19 05:00:00 UTC-02
date: parsed relative part: +1 minutes
date: input timezone: parsed date/time string (-02)
date: using specified time as starting value: '05:00:00'
date: starting date/time: '(Y-M-D) 2019-01-19 05:00:00 TZ=-02'
date: '(Y-M-D) 2019-01-19 05:00:00 TZ=-02' = 1547881200 epoch-seconds
date: after time adjustment (+0 hours, +1 minutes, +0 seconds, +0 ns),
date: new time = 1547881260 epoch-seconds
date: timezone: TZ="UTC" environment value
date: final: 1547881260.000000000 (epoch-seconds)
date: final: (Y-M-D) 2019-01-19 07:01:00 (UTC)
date: final: (Y-M-D) 2019-01-19 07:01:00 (UTC+00)
2019-01-19_07:01:00UTC
So, well, date arithmetic is being done, just not the one that we asked for. ¯\(ツ)/¯
Best Answer
With the GNU implementation of
date
, to display yesterday's date, enter:OR
For your question:
OR
For using it with variables, you can use
$()
:For general case
n
days and for a specific date:Source