| eval duedate1 = strftime(strptime(duedate,"%Y-%m-%d"),"%Y-%m-%d %H:%M:%S")
| eval current = strftime(now(),"%Y-%m-%d %H:%M:%S")
| eval daysdiff=round((current-duedate1)/86400,0)
| table current duedate1 daysdiff
Differences between timestamps must be calculated using the epoch form of timestamp. Try this:
| eval duedate1 = strptime(duedate,"%Y-%m-%d")
| eval current = now()
| eval daysdiff=round((current-duedate1)/86400,0)
| eval today = strftime(current, "%Y-%m-%d %H:%M:%S")
| table today duedate daysdiff