Looks like you're right, the value is in microseconds.
FYI: here's the Zimbra install's version info:
Code:
Name : zimbra-core Relocations: (not relocatable)
Version : 4.5.6_GA_1044.RHEL4 Vendor: Zimbra, Inc.
Release : 20070706161941 Build Date: Fri 06 Jul 2007 06:23:54 PM CDT
Install Date: Thu 12 Jul 2007 06:53:59 AM CDT Build Host: build.liquidsys.com
Group : Applications/Messaging Source RPM: zimbra-core-4.5.6_GA_1044.RHEL4-20070706161941.src.rpm