I mean it doesn't show the instances of the old jobs from several years ago, which is fine because I'm not too bothered about Job Activity Monitor anyway.

For example a job which runs daily and is in sysjobactivity as starting in 2010 but never finished will just show as normal for it's day to day routine in Job Activity Monitor. If it ran this morning and succeeded, then Job Activity Monitor say's it succeeded and the last run date was this morning. There is no mention of the 2010 instance of it in Job Activity Monitor.

The real questions I'm getting at though, are why have these old jobs never been marked with an end date, and can I delete these old entries from sysJobActivity, or UPDATE them with an end date of a long long time ago, so that they will never play into any queries. Can anyone think of anything that might break by doing this?