# date -d "$(awk -F. '{print $1}' /proc/uptime) second ago" +"%Y-%m-%d %H:%M:%S"
2021-12-18 21:55:56
# cat /proc/uptime
2144676.83 2131002.93
# date -d "$(awk -F. '{print $1}' /proc/uptime) second ago"
Sat Dec 18 21:55:56 HKT 2021
# date -d "20211003 +90 day"
Sat Jan  1 00:00:00 HKT 2022
# date -d "20211003 +90 days"
Sat Jan  1 00:00:00 HKT 2022
# date -d "- $(awk -F. '{print $1}' /proc/uptime) second" +"%Y-%m-%d %H:%M:%S"    
2021-12-18 21:55:55
# date
Wed Jan 12 17:44:15 HKT 2022
# date -d "+90 days"
Tue Apr 12 17:44:50 HKT 2022