Am I the only one with incorrect boot time for Cisco devices?

This is a limitation of the SNMP counter for uptime, it rolls over every 500ish days .  You can also reset the counter by stopping the snmp service.  It's just kind of a lie that the field is called sysUptime, but thats par for the course with SNMP data.

networkengineering.stackexchange.com/.../what-is-the-length-of-snmp-sysuptime-counter

There's not really a great workaround, some vendors do some trickery where if it looks like it rolled over they internally keep a counter of the number of times they have seen it roll over, but even thats kind of a lie too because it doesn't know about rollovers that may have happened before the system was added to the tool.

Like you saw, hrsysuptime is a different counter, but is not part of the standard set most device manufacturers offer so it rarely works.  

If you have NCM you could capture the device output for "show version" but it would take some massaging to turn that into a sortable list that you can export.

I have created a UnDP for snmpEngineTime (1.3.6.1.6.3.10.2.1.3), which in most cases matches the number we are looking for.

I also created a Transform for snmpEngineTimeDays ({snmpEngineTime} / 86400) to help display on reports.

thanks for the info, it took some research to find a workable alternative

Most all Cisco devices return a useful value for this OID, but sadly- the Cisco wireless controllers in my environment do not