Time synchronization in your network should always be configured, but sometimes users underestimate its importance because they believe that having the network time-synced is not that important. It is, however, critical. If the ESXi hosts are not in sync, you might face some communication issues between vSphere components that could cause a service outage. If you use AD in your network, for example, the Domain Controllers (DCs) and clients must be time-synced to avoid authentication problems. If the time between the DCs and clients differs by more than five minutes, Kerberos tickets will fail, and you will not be able to log in. By default, machines joined to a domain will contact the DC that holds the Primary Domain Controller (PDC) emulator role to synchronize the time.

If your network is not time-synced, you may experience authentication issues between vCenter Server and the Platform Services Controller (PSC). When vSphere components are not time-synced, the login procedure may fail due to communication issues between the PSC and vCenter.

VMs use VMware Tools to synchronize the time with the host. Although a VM can be time-synced with the ESXi host using VMware Tools (VMs automatically synchronize the time when specific events occur, such as VM vMotion, snapshot creation, or guest OS reboots), it is recommended to synchronize the guest OS time with the NTP source instead. 

To keep the time synchronized, ESXi supports the NTP, which you can configure through the vSphere Client. As a time source for your network, you should use a reliable external source, such as the pool.ntp.org project (a big virtual cluster of time servers providing a reliable, easy-to-use NTP service) or an internal source, such as a DC synchronized with an external time source.

Let's take a look at how to configure an NTP in your ESXi by performing the following steps:

1. Open the vSphere Client by typing the address, https://<ESXi_IP>/ui into your favorite browser, and log in to the host.
2. In the Navigator, select Manage. Go to the System tab and select Time & date.
3. Click Edit settings to open the time configuration window.
4. Select Use Network Time Protocol (enable NTP client) to specify the NTP parameters.
5. Select Start and stop with port usage (the recommended option) in the NTP service startup policy drop-down menu. In the NTP servers field, enter the NTP server to use. Specify the pool.ntp.org NTP servers to point the host to an external source directly, or enter the AD DC that holds the PDC emulator role configured to synchronize the time to an external source, to ensure the correct time.
6. Click Save to save the configuration.
7. Click Action and select NTP service | Start to start the service.

The time of the ESXi host is now synchronized with a reliable NTP server.