[BASH] store UNIX epoch time in variable

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Linux

shell

Bash

I need to compare today's date in Epoch time to another Epoch date stored in a variable.

I can store today's epoch in a variable with no problem. I run into trouble when I try to store a derived $cert_experation_epoch.

This is most likely a simple issue of redirection but I'm a bit stuck.

What I have tried:

date
Sat Oct 16 11:42:42 PDT 2021
todays_epoch=`date "+%s"`
echo $todays_epoch
1634408601

echo $cert_expiration
Oct 4 2022
cert_expiration_epoch=`date "+%s" <<< $cert_expiration`

echo $cert_expiration_epoch
1634408601  < ------ This is wrong and it looks like it's just doing the date command without redirecting the output into the variable cert_expiration_epoch.