I don't recall a direct way of getting such a report, but there're at least two alternatives that could get you the uptime of all devices:

1) You could run a Detailed Device Report in RME against all devices. Uptime is included, along with a whole enchilada of other info for each device. This number is based on the SNMP OID value of sysUpTime (.1.3.6.1.2.1.1.3.0) in LMS 2.6, I think. This means it gets affected by CSCdy02471, due to the value's 32-bit nature, meaning it'll wrap around to zero if the actual uptime is 1 year, 18 weeks, 6 days, 2 hours, 27 minutes, 52 seconds, and 95 hundreths of a second. In later versions of LMS, this number may be corroborated by RME via double-checking snmpEngineTime (.1.3.6.1.6.3.10.2.1.3.0), but I don't recall in which exact release.

2) Another way is to simply schedule a NetShow job to run the "show version" command against all devices. The uptime obtained that way is not subject to the aforementioned CSCdy02471 SNMP counter rollover.

Either output will require further parsing to get the over/under of one year.