Compare Current date with field Date

[dlnewman]

I am trying to compare the current date with the lastInformTime I have tried | eval but nothing seems to work. 

index="device_list" pppUsername=* provRecordStatus=Succeeded
| eval timenow=now()
| spath lastInformTime
| search lastInformTime>=timenow
| dedup macAddress, serialNumber
| table ipAddress, serialNumber, lastInformTime, pppUsername, macAddress

The _time that is brought in during the import does not compare with any date in the export. I am not sure where Splunk is getting it from. Is there a way to set the _time to the lastInformTime? TIA

[to4kawa]

| eval _time=lastInformTime

or

| rename lastInformTime as _time