How to get the latest git commit time of another repository from a Jenkins declarative pipeline - git-commit

Question

I have a declarative pipeline. In this pipeline I want to add a verify step to check the last code commit time which lies in another repository. Based on the commit time, I do have to proceed with the next steps.
I am facing issues in fetching the commit time. I am very new to git commands, so unable to resolve the issue.
Points - My Jenkins file is in one repository, and I need the commit details of another repository. I need specifically the commit time (Epoc)
My code looks like this -
#Library('xx-libs') _
pipeline {
agent any
options {
timeout(time: 2, unit: 'HOURS')
}
parameters {
xxxxx
}
environment {
xxxxxx
}
stages {
stage('Checkout source code') {
steps {
script {
checkout ([$class: 'GitSCM',
branches: [[name: '*/'+branch]],
extensions: [
[$class: 'CloneOption', noTags: true, timeout: 20],
[$class: 'RelativeTargetDirectory', relativeTargetDir: '/tmp/core/'],
[$class: 'SparseCheckoutPaths', sparseCheckoutPaths:[[$class:'SparseCheckoutPath', path:'code/System/Infra/Version/version.cpp']]],
[$class: 'CleanBeforeCheckout']
],
userRemoteConfigs: [[
credentialsId: 'azure-bearer-auto-updated',
url: 'https://xx.xx.com/xxx/xxx/_git/core'
]]
])
git_time = sh script: "git show -s --format=%ct", returnStdout: true
echo "$git_time"
}
}
}
} // stages
}