Zabbix agent 1.8.10 compiled as a 64-bit binary on SPARC Solaris 10, with Sun C 5.11 SunOS_sparc 145354-05 2011/09/13 (Studio 12.2).
Oracle Solaris 10 8/11 s10s_u10wos_17b SPARC, Generic_147440-05 sun4v

The problem seems to manifest depending on whether the agent is compiled as a 32-bit or a 64-bit binary:

$ CFLAGS="$CFLAGS -m32" ./configure --enable-agent ...
...
$ src/zabbix_agent/zabbix_agentd -t system.boottime
system.boottime                               [u|1372661428]
$ src/zabbix_agent/zabbix_agentd -t system.uptime
system.uptime                                 [u|8132082]

$ CFLAGS="$CFLAGS -m64" ./configure --enable-agent ...
...
$ src/zabbix_agent/zabbix_agentd -t system.boottime
system.boottime                               [u|5895535941740658688]
$ src/zabbix_agent/zabbix_agentd -t system.uptime
system.uptime                                 [u|12551208133349686272]

The bug seems to have been there since the very beginning in 2005 (see r2198). Although what ZBX-6988 is concerned, it only started to manifest on a regular basis after r37064:

$ svn log ^/ -c 37064
------------------------------------------------------------------------
r37064 | igors | 2013-07-17 14:25:07 +0300 (Wed, 17 Jul 2013) | 2 lines

....I..... [ZBX-6637] added -m64 compilation flag for solaris to be able to understand all attributes of 64-bit processes

------------------------------------------------------------------------

Fixed in development branches:

• svn://svn.zabbix.com/branches/dev/ZBX-4658 (for 1.8)
• svn://svn.zabbix.com/branches/dev/ZBX-4658-2.0 (for 2.0 and trunk)

Structure kstat_named_t has a member data_type (see http://www.manpages.spotlynx.com/solaris/man/kstat.3KSTAT), which describes the data type of the stored value. So it was decided to create a function, get_kstat_numeric_value(), which would return the value through the appropriate member of the union. Previously access to the union's value was hardcoded: not just for system.boottime and system.uptime, but in other places, too.

Branches 1.8 and 2.0 successfully tested.
Illumos source code was used for solution code review.
You can search Illumos source code online at http://src.illumos.org/source/s?path=usr/src&project=illumos-gate.
If you want to download all Illumos source follow instructions at http://wiki.illumos.org/display/illumos/How+To+Build+illumos.

Fixed in pre-1.8.19 r39228, pre-2.0.10 r39230 and pre-2.1.8 (trunk) r39231.