Cox Proportional Hazard with missing covariate data

I concur with your other reply - you have left-trunctated data.
 
 Left-truncated: Any parts that failed before 2001 (start of study) are not 
included in the study.
 
  Left-censored: Parts that failed before the study start are included, but with 
an indeterminate failure date ("< 20001").  
  
  The Cox model deals with left truncated data easily, but not with left 
censored.  In the Cox model all that matters are covariate values at the time 
that an event occurs, so an unknown past is not an issue. 
  
  	Terry Therneau