Bash: Offset a date by n seconds

Question

I am trying to offset a time in the following format. I am unable to get this working. Any suggestions?

This doesn't work

DATE0="26Sep21 06:10:14"
DATE1=$(date -d "$DATE0 + 1 seconds"  +'%d%b%y %H:%M:%S'); 
echo $DATE1

This doesn't work either

n=10
DATE0="26Sep21 06:10:14"
DATE1=$(date -d "$DATE0 + $n seconds"  +'%d%b%y %H:%M:%S'); 
echo $DATE1

Answer 1

# Init
n=10
DATE0="26Sep21 06:10:14"

# Seconds since 01/01/1970
SECONDS=$(date +%s --date "${DATE0}")

# Add seconds
SECONDS=$(( SECONDS + n ))

# Reformat seconds
DATE1=$(date --date "@${SECONDS}" +'%d%b%y %H:%M:%S')

One line:

# Init
n=10
DATE0="26Sep21 06:10:14"

# Compute
DATE1=$(date --date "@$(( $(date +%s --date "${DATE0}") + n ))" +'%d%b%y %H:%M:%S')