If you just want to know whether the certificate has expired (or will do so within the next N seconds), the -checkend <seconds> option to openssl x509 will tell you:

if openssl x509 -checkend 86400 -noout -in file.pem
then
  echo "Certificate is good for another day!"
else
  echo "Certificate has expired or will do so within 24 hours!"
  echo "(or is invalid/not found)"
fi

This saves having to do date/time comparisons yourself.

openssl will return an exit code of 0 (zero) if the certificate has not expired and will not do so for the next 86400 seconds, in the example above. If the certificate will have expired or has already done so - or some other error like an invalid/nonexistent file - the return code is 1.

(Of course, it assumes the time/date is set correctly)

Be aware that older versions of openssl have a bug which means if the time specified in checkend is too large, 0 will always be returned (https://github.com/openssl/openssl/issues/6180).

#!/bin/bash

# Based on https://askubuntu.com/questions/1198619/bash-script-to-calculate-remaining-days-to-expire-ssl-certs-in-a-website

### Read Site Certificate and save as File ###
echo -n | openssl s_client -servername $1 -connect $1:443 2>/dev/null | sed -ne '/-BEGIN CERTIFICATE-/,/-END CERTIFICATE-/p' > $1.crt


### Get Full Expiratoin Date ###
date=$(openssl x509 -in $1.crt -enddate -noout | sed "s/.*=\(.*\)/\1/" | awk -F " " '{print $1,$2,$3,$4}')

### Convert Expiration Date in Epoch Format ###
date_s=$(date -j -f "%b %d %T %Y" "$date" "+%s")

### Get Curent Date in Epoch Format ###
now_s=$(date +%s)

### Calculate Time Difference ###
date_diff=$(( (date_s - now_s) / 86400 ))

echo "Certificate for $1 will expire in $date_diff days"