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
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