Cannot backup exchange error log shows time disparity

The clocks on the exchange server, AD & DC might be out by more than 6 minutes. Change the time so they are all the same. You can set NTP time as follows;

By default, the clients computers get their time from a Domain Controller and the Domain Controller gets his time from the domain’s PDC Operation Master. Therefore the PDC must synchronize his time from an external source. I usually use the servers listed at the NTP Pool Project website. Before you begin, don’t forget to open the default UDP 123 port (in- and outbound) on your (corporate) firewall.

1. First, locate your PDC Server. Open the command prompt and type: C:>netdom /query fsmo
2. Log in to your PDC Server and open the command prompt.
3. Stop the W32Time service: C:>net stop w32time
4. Configure the external time sources, type: C:> w32tm /config /syncfromflags:manual /manualpeerlist:”0.pool.ntp.org, 1.pool.ntp.org, 2.pool.ntp.org”
5. Make your PDC a reliable time source for the clients. Type: C:>w32tm /config /reliable:yes
6. Start the w32time service: C:>net start w32time
7. The windows time service should begin synchronizing the time. You can check the external NTP servers in the time configuration by typing: C:>w32tm /query /configuration
8. Check the Event Viewer for any errors.