I found this https://stackoverflow.com/questions/57364453/pass-date-command-as-parameter-to-kubernetes-cronjob which is similar, but did not solve my problem.

I'm trying to backup etcd using a cronjob, but etcd image doesn't have "date" command.

apiVersion: batch/v1beta1
kind: CronJob
metadata:
  name: backup
  namespace: kube-system
spec:
  concurrencyPolicy: Allow
  failedJobsHistoryLimit: 1
  jobTemplate:
    spec:
      template:
        spec:
          containers:
          - args:
            - -c
            - etcdctl --endpoints=https://127.0.0.1:2379 --cacert=/etc/kubernetes/pki/etcd/ca.crt
              --cert=/etc/kubernetes/pki/etcd/healthcheck-client.crt --key=/etc/kubernetes/pki/etcd/healthcheck-client.key
              snapshot save /backup/etcd-snapshot-$(DATE_CURR).db
            command:
            - /bin/sh
            env:
            - name: ETCDCTL_API
              value: "3"
            - name: DATE_CURR
              #value: $(date --date= +"%Y-%m-%d_%H:%M:%S_%Z")
              value: $(date +"%Y-%m-%d_%H:%M:%S_%Z")
            image: k8s.gcr.io/etcd:3.4.13-0
            imagePullPolicy: IfNotPresent
            name: backup
            resources: {}
            terminationMessagePath: /dev/termination-log
            terminationMessagePolicy: File
            volumeMounts:
            - mountPath: /etc/kubernetes/pki/etcd
              name: etcd-certs
              readOnly: true
            - mountPath: /backup
              name: backup
          - args:
            - -c
            - find /backup -type f -mtime +30 -exec rm -f {} \;
            command:
            - /bin/sh
            env:
            - name: ETCDCTL_API
              value: "3"
            image: k8s.gcr.io/etcd:3.4.13-0
            imagePullPolicy: IfNotPresent
            name: cleanup
            resources: {}
            terminationMessagePath: /dev/termination-log
            terminationMessagePolicy: File
            volumeMounts:
            - mountPath: /backup
              name: backup
          dnsPolicy: ClusterFirst
          hostNetwork: true
          nodeName: homelab-a
          restartPolicy: OnFailure
          schedulerName: default-scheduler
          securityContext: {}
          terminationGracePeriodSeconds: 30
          volumes:
          - hostPath:
              path: /etc/kubernetes/pki/etcd
              type: DirectoryOrCreate
            name: etcd-certs
          - hostPath:
              path: /opt/etcd_backups
              type: DirectoryOrCreate
            name: backup
  schedule: 0 */6 * * *
  successfulJobsHistoryLimit: 3
  suspend: false

when this runs, it produces a file named "etcd-snapshot-.db", no date. If I can catch the logs, it says that "date" is not a known command. Sure enough, when I am able to exec into the pod while it's running, date does not work in the etcd image. How can I pass the date from the system as a variable so that it just uses the text and not try to actually invoke the "date" command?

Edit: Thanks to @Andrew, here is the solution:

apiVersion: batch/v1beta1
kind: CronJob
metadata:
  name: backup
  namespace: kube-system
spec:
  concurrencyPolicy: Allow
  failedJobsHistoryLimit: 1
  jobTemplate:
    spec:
      template:
        spec:
          containers:
          - args:
            - -c
            - etcdctl --endpoints=https://127.0.0.1:2379 --cacert=/etc/kubernetes/pki/etcd/ca.crt
              --cert=/etc/kubernetes/pki/etcd/healthcheck-client.crt --key=/etc/kubernetes/pki/etcd/healthcheck-client.key
              snapshot save /backup/etcd-snapshot-$(echo `printf "%(%Y-%m-%d_%H:%M:%S_%Z)T\n"`).db
            command:
            - /bin/sh
            env:
            - name: ETCDCTL_API
              value: "3"
            image: k8s.gcr.io/etcd:3.4.13-0
            imagePullPolicy: IfNotPresent
            name: backup
            resources: {}
            terminationMessagePath: /dev/termination-log
            terminationMessagePolicy: File
            volumeMounts:
            - mountPath: /etc/kubernetes/pki/etcd
              name: etcd-certs
              readOnly: true
            - mountPath: /backup
              name: backup
          - args:
            - -c
            - find /backup -type f -mtime +30 -exec rm -f {} \;
            command:
            - /bin/sh
            env:
            - name: ETCDCTL_API
              value: "3"
            image: k8s.gcr.io/etcd:3.4.13-0
            imagePullPolicy: IfNotPresent
            name: cleanup
            resources: {}
            terminationMessagePath: /dev/termination-log
            terminationMessagePolicy: File
            volumeMounts:
            - mountPath: /backup
              name: backup
          dnsPolicy: ClusterFirst
          hostNetwork: true
          nodeName: homelab-a
          restartPolicy: OnFailure
          schedulerName: default-scheduler
          securityContext: {}
          terminationGracePeriodSeconds: 30
          volumes:
          - hostPath:
              path: /etc/kubernetes/pki/etcd
              type: DirectoryOrCreate
            name: etcd-certs
          - hostPath:
              path: /opt/etcd_backups
              type: DirectoryOrCreate
            name: backup
  schedule: 0 */6 * * *
  successfulJobsHistoryLimit: 3
  suspend: false