Create an snmp template, apply it to a host with a very high uptime, create item for uptime with 's' as units, let zabbix perform automatic conversion to days+hours, converted value seems to be 240 days less than actual.

example:
This is the snmp oid I grab:
DISMAN-EVENT-MIB::sysUpTimeInstance = Timeticks: (2212130098) 256 days, 0:48:20.98

It gets put into db correctly after multiplying by 0.01 to convert timeticks to seconds:
System Uptime 22126018.5

now, specify 's' as units:
System Uptime 8m 16d 2h

Zabbix is reporting the uptime as 16 days instead of 256 days.
This occurs regardless of weather unsigned int or float or used to hold datatype.

To verify the correct result should be 256:
matt@hyperion ~/dev/gentooZabbixTemplates $ bc
bc 1.06.95
Copyright 1991-1994, 1997, 1998, 2000, 2004, 2006 Free Software Foundation, Inc.
This is free software with ABSOLUTELY NO WARRANTY.
For details type `warranty'.
22121300 / 60 / 60 / 24
256