bash if greater than or equal to error

With the bash script below I am getting an error;

line 16: [: : integer expression expected but I can't figure out why? The error lies in if [ "$(log_date)" -le "$days_in_ms" ]; then Can anyone help and explain as BASH if statements confuse me

#!/bin/bash

region="eu-west-1"
retention_days="28"
days_in_ms="$(date +%s%3N --date=-"$retention_days"+days)"

log_date () {
  aws logs describe-log-streams --region $region --log-group-name "$groups" | jq -r '.logStreams[].lastEventTimestamp' > /dev/null 2>&1
}
log_name () {
  aws logs describe-log-streams --region $region --log-group-name "$groups" | jq -r '.logStreams[].logStreamName' > /dev/null 2>&1
}

mapfile -t logGroups < <(aws logs describe-log-groups --region $region | jq -r '.logGroups[].logGroupName') > /dev/null 2>&1
for groups in "${logGroups[@]}"; do
    if [ "$(log_date)" -le "$days_in_ms" ]; then
        log_name
     fi
done