Construct daily 6 hourly date series (4 time-steps a day) using linux "date" command

Question

I am trying to construct a daily 6-hourly date series for some 'N' number of years. I tried to do like this

for i in {1..1825}; do 
date -d "00:00 01-01-1998 +6 hours" +"%d-%m-%Y %H:%M"
done

But this is not what I wanted.

My goal is to create like this in order;

01-01-1998 00:00 
01-01-1998 06:00 
01-01-1998 12:00 
01-01-1998 18:00 
02-01-1998 00:00 
02-01-1998 06:00
02-01-1998 12:00 
02-01-1998 18:00 
.
.
.etc

How can I do this using the date command?

Any help is highly appreciated. Thanks in advance.

Answer 1

You have to worry about daylight saving time. For example, my time zone is Canada/Eastern, and:

$ d=$(date -d "2020-03-08 00:00:00")
$ echo $d
Sun Mar 8 00:00:00 EST 2020
$ d=$(date -d "$d + 6 hours")
$ echo $d
Sun Mar 8 07:00:00 EDT 2020
# ........^^

and

$ d=$(date -d "2020-11-01 00:00:00")
$ echo $d
Sun Nov 1 00:00:00 EDT 2020
$ d=$(date -d "$d + 6 hours")
$ echo $d
Sun Nov 1 05:00:00 EST 2020
# ........^^

Make sure you specify UTC for the time zone.

$ d=$(date -u -d "2020-11-01 00:00:00")
$ echo $d
Sun Nov 1 00:00:00 UTC 2020
$ d=$(date -u -d "$d + 6 hours")
$ echo $d
Sun Nov 1 06:00:00 UTC 2020
# ........^^

So:

fmt='%d-%m-%Y %H:%M'
d=$(date -u -d "1998-01-01 00:00:00")
for i in {1..1825}; do 
    # print the current date
    date -u -d "$d" "+$fmt"
    # add 6 hours
    d=$(date -u -d "$d + 6 hours")
done

I would strongly recommend you use the ISO date format YYYY-mm-dd: that is an unambiguous date format, and it sorts the same lexically and chronologically.

Answer 2

You can save the date of every calculation in a variable and use it to calculate the next date. I use the default format in standard locale for storing the date because date cannot parse your specified format.

# start date
d=$(LC_ALL=C date -d "1998-01-01 00:00")
echo "# d=$d"
# convert format
date -d "$d" +"%d-%m-%Y %H:%M"

for i in {1..1825}; do 
    # add 6 hours
    d=$(LC_ALL=C date -d "$d +6 hours")
    # convert format
    date -d "$d" +"%d-%m-%Y %H:%M"
done

This prints

# d=Thu Jan  1 00:00:00 CET 1998
01-01-1998 00:00
01-01-1998 06:00
01-01-1998 12:00
01-01-1998 18:00
02-01-1998 00:00
02-01-1998 06:00
02-01-1998 12:00
02-01-1998 18:00
03-01-1998 00:00
03-01-1998 06:00
03-01-1998 12:00
03-01-1998 18:00
04-01-1998 00:00
04-01-1998 06:00
...

The results depend on your time zone and daylight saving time rules.

Your starting time specification may be ambiguous because it doesn't specify the time zone.

Edit based on Paul_Pedant's comment:

Make an outer loop for + $i days, and an inner loop hard-coded 00, 06, 12, 18. It runs a quarter as many date processes, and it does not care about DST variations.

# start date
d=$(LC_ALL=C date -d "1998-01-01 00:00")
echo "# d=$d"

for i in {1..456}; do 
    # convert (partial) format
    out=$(date -d "$d" +"%d-%m-%Y")
    for h in 00 06 12 18; do
        echo "$out $h:00"
    done
    # add 1 day
    d=$(LC_ALL=C date -d "$d +1 day")
done

Please specify in the question what result you expect when switching between normal time and daylight saving time occurs. (strictly printing 00:00, 06:00, 12:00, 18:00 or exactly 6 hours time difference when switching DST)

Edit based on glenn jackman's answer

Adding option -u to all date calls will make the script nearly the same as written by glenn jackman and result in strictly printing 00:00, 06:00, 12:00, 18:00 regardless of DST.

Answer 3

With GNU date you can add an ever-increasing number of hours to your starting point

start="$(date +'%Y-%m-%d')"
for i in {0..1825}
do
    date --date "$start +$((i*6)) hours" +'%d-%m-%Y %H:%M'
done

The six hour interval remains constant but as a result in my locale (UK timezone) the actual times shift as a consequence of the switch between summer time and winter time.

24-10-2020 00:00
24-10-2020 06:00
24-10-2020 12:00
24-10-2020 18:00
25-10-2020 00:00
25-10-2020 05:00    ← Six hours on from midnight because the clock goes back one hour at 03:00
25-10-2020 11:00
25-10-2020 17:00
25-10-2020 23:00

It's not specified in your question whether you want the intervals to be at midnight, 6am, noon and 6pm regardless of any possible switch between summer/winter time, or you want the intervals to be fixed at precise six hour intervals.

Use date --utc instead of just date to guarantee generating a series of datetimes that appear to be six hours apart without regard to the timezone.