If it's off by exactly 13 hours then it sounds like it's more likely a time zone issue than an ntp issue.
Have you followed the procedure in the configuration guide?
For the FMC:
Step 1 Manually set the system time on the Firepower Management Center:
a) Choose System > Configuration.
b) Click Time Synchronization.
c) If Serve Time via NTP is Enabled, choose Disabled.
d) Click Save.
e) For Set My Clock, choose Manually in Local Configuration.
f) Click Save.
g) In the navigation panel at the left side of the screen, click Time.
h) Use the Set Time drop-down lists to set the time.
i) If the time zone displayed is not UTC, click it and set the time zone to UTC.
j) Click Save.
k) Click Done.
l) Click Apply.
Step 2 Set the Firepower Management Center to serve as an NTP server:
a) In the navigation panel at the left side of the screen, click Time Synchronization.
b) For Serve Time via NTP, choose Enabled.
c) Click Save.
Step 3 Set managed devices to synchronize with the Firepower Management Center NTP server:
In the Time Synchronization settings for the platform settings policy assigned to your managed devices, set
the clock to synchronize Via NTP from Management Center and deploy the change to managed devices.
For the managed device:
Step 1 Choose Devices > Platform Settings.
Step 2 Create or edit a Firepower policy.
Step 3 Click Time Synchronization.
Step 4 Specify how time is synchronized on Classic managed devices:
• Choose Via NTP from Management Center if your Firepower Management Center is configured to
serve as an NTP server.
