Placing a conditional return inside a bash function

I know that its possible to have a return within an if [ ] statement. However, the return (I think) must be an integer, and I need to return a string. For example:

#Function returns a date value formatted %Y-%m-%d
function chartStartsAt () {

    CHART_START=`jq 'map(.closedDate | values) | min' < output/json/FullIssueList.json`;
    CHART_START="${CHART_START//\"}"

    #Check if the date is null, if so return todays date -1
    if [ -z CHART_START ]
    then
        echo "date -v-1d +%Y-%m-%d";
    else
        echo "${CHART_START%T*}";
    fi

}