Date :
Part of gnu/linux coreutils
I’ve some tricks and basic knowledge around date cli I want to share it with our members here at forums, it might be useful for user whose spend 80% of time in terminal,
Keep in mind that date util has calendar awareness it knows leap year
and aware of months with 28/31 days etc.
1
change the system clock
date +%T -s "10:13:13" # set time 24H
date -s "11:48:00 PM" # set time 12H
sudo date -s "2026-02-01 00:00:00" # set date and time
# use hwclock to write/read -> cmos
2
count duration between two dates
echo $(( ($(date -d 2026-1-1 +%s) - $(date -d 2025-1-1 +%s)) / 86400 )) # Days
echo $(( ($(date -d 2026-1-1 +%s) - $(date -d 2025-1-1 +%s)) / 604800 )) # Weeks
Chronosphere ||| count everything in between 
d1="2025-01-01" d2="2026-01-01"; s=$(( $(date -d "$d2" +%s) - $(date -d "$d1" +%s) )); echo "Days: $(( s / 86400 )) Weeks: $(( s / 604800 )) Months: $(( ( $(date -d "$d2" +%Y) - $(date -d "$d1" +%Y) ) * 12 + $(date -d "$d2" +%m) - $(date -d "$d1" +%m) )) Years: $(( $(date -d "$d2" +%Y) - $(date -d "$d1" +%Y) ))"
3
Print day name from date
date -d "2026-02-28" +"%A"
4
print the date from 3 days ago
date --date='3 days ago'
print the date after 42 days
date --date='42 days'
print date after 5 months and 2 days
date --date='5 months 2 day'
Unix Timestamp
date +%s
Thanks for reading , &I hope you learn something new 
