Add systemd-timesyncd setup instructions for Debian 13

Added instructions for using systemd-timesyncd on Debian 13 and later, including enabling NTP synchronization, configuring trusted NTP servers, and checking synchronization status.

README.md

@@ -1015,6 +1015,76 @@ NTP stands for Network Time Protocol. In the context of this guide, an NTP clien
 
 #### Steps
 
+##### Debian 13 (Trixie) and later: systemd-timesyncd
+
+`systemd-timesyncd` is a lightweight SNTP client that is already included in Debian. Unlike the full `ntpd` daemon, it does not listen on any port, which makes it a smaller attack surface. For the purposes of this guide - keeping your server's clock in sync - it is all you need.
+
+1. Enable NTP synchronization:
+
+    ``` bash
+    sudo timedatectl set-ntp true
+    ```
+
+1. Verify it is working:
+
+    ``` bash
+    timedatectl status
+    ```
+
+    You should see `NTP service: active` and `System clock synchronized: yes` in the output.
+
+1. Configure trusted NTP servers. Make a backup of the configuration file and then edit it:
+
+    ``` bash
+    sudo cp --archive /etc/systemd/timesyncd.conf /etc/systemd/timesyncd.conf-COPY-$(date +"%Y%m%d%H%M%S")
+    ```
+
+    Edit `/etc/systemd/timesyncd.conf` and uncomment/set the `[Time]` section:
+
+    ```
+    [Time]
+    NTP=pool.ntp.org
+    FallbackNTP=0.debian.pool.ntp.org 1.debian.pool.ntp.org 2.debian.pool.ntp.org
+    ```
+
+    [For the lazy](#editing-configuration-files---for-the-lazy):
+
+    ``` bash
+    sudo sed -i -r -e "s/^#?NTP=.*$/NTP=pool.ntp.org         # added by $(whoami) on $(date +"%Y-%m-%d @ %H:%M:%S")/" /etc/systemd/timesyncd.conf
+    sudo sed -i -r -e "s/^#?FallbackNTP=.*$/FallbackNTP=0.debian.pool.ntp.org 1.debian.pool.ntp.org 2.debian.pool.ntp.org         # added by $(whoami) on $(date +"%Y-%m-%d @ %H:%M:%S")/" /etc/systemd/timesyncd.conf
+    ```
+
+1. Restart the service to apply the changes:
+
+    ``` bash
+    sudo systemctl restart systemd-timesyncd
+    ```
+
+1. Check the synchronization status:
+
+    ``` bash
+    timedatectl timesync-status
+    ```
+
+    > ```
+    >        Server: 108.61.56.35 (pool.ntp.org)
+    > Poll interval: 32s (min: 32s; max: 34min 8s)
+    >          Leap: normal
+    >       Version: 4
+    >       Stratum: 2
+    >     Reference: C342F10A
+    >     Precision: 1us (2^0)
+    >  Root distance: 24.054ms (max: 5s)
+    >        Offset: +2.156ms
+    >         Delay: 48.567ms
+    >        Jitter: 1.452ms
+    >  Packet count: 3
+    > ```
+
+##### Debian 12 (Bookworm) and earlier: ntp package
+
+> **Note:** These steps apply to **Debian 12 and earlier** only. On Debian 13+, the `ntp` package is no longer available -- use the [systemd-timesyncd steps](#debian-13-trixie-and-later-systemd-timesyncd) above instead.
+
 1. Install ntp.
 
     On Debian based systems: