Linux date command, finding seconds to next hour

Question

How do I find the seconds to the next hour using date? I know I can do

date -d "next hour"

but that just adds 1 hour to the present time. I want it to show the seconds to the next full hour. For example if the current time is 9:39am I want to find the number of seconds to 10am

Answer 1

The epoch timestamp of right now is

now=$(date '+%s')

That of the next hour is

next=$(date -d $(date -d 'next hour' '+%H:00:00') '+%s')

The number of seconds until the next hour is

echo $(( next - now ))

For a continuous solution, use functions:

now() { date +%s; }
next() { date -d $(date -d "next ${1- hour}" '+%H:00:00') '+%s'; }

And now you have

echo $(( $(next) - $(now) ))

and even

echo $(( $(next day) - $(now) ))

Another way

Another slightly mathier approach still uses the epoch timestamp. We know it started on an hour, so the timestamp mod 3600 only equals zero on the hour. Thus

$(( $(date +%s) % 3600 ))

is the number of seconds since the last hour, and

$(( 3600 - $(date +%s) % 3600 ))

is the number of seconds until the next.