find files older than today and zip them up with the date as part of the extension

1. find files older than today in a given directory with a given extension

$ find -mtime +1 ${DIR} -name "*.${EXT}"

2. calculate the last modified time (seconds since epoch)

$ MOD_SECS=$(stat -c%Y ${FILE})

3. convert the seconds since epoch into a human readable date format

$ MOD_DATE=$(date +\%Y-\%m-\%d --date="@${MOD_SECS}")

4. create a gzip file with the suffix including the date when the file was last modified

$ gzip -S .${MOD_DATE}.gz ${FILE}

5. putting it all together

for FILE in $(find ${DIR} -mtime +1 -name "*.${EXT}"); do
    MOD_SECS=$(stat -c%Y ${FILE})
    MOD_DATE=$(date +\%Y-\%m-\%d --date="@${MOD_SECS}")
    gzip -S .${MOD_DATE}.gz ${FILE}
done

6. as a script:

#!/bin/bash

if [ "$#" -ne 2 ]; then
    echo "Usage: $0 dir ext"
    exit 1
fi

DIR=$1
EXT=$2

for FILE in $(find ${DIR} -mtime +1 -name "*.${EXT}"); do
    MOD_SECS=$(stat -c%Y ${FILE})
    MOD_DATE=$(date +\%Y-\%m-\%d --date="@${MOD_SECS}")
    gzip -S .${MOD_DATE}.gz ${FILE}
done