As of the 2020 SIPP, the calculation of TRESDUR has been updated to correct an error affecting Wave 3+ cases. The error has previously been noted in 2014 SIPP Panel Waves 3 and 4. Separate user notes about 2014 SIPP Panel Waves 3 and 4 are accessible via the following links:

2014 SIPP Panel Wave 3: Incorrect Duration of Residence Values in 2014 (census.gov)

2014 SIPP Panel Wave 4: Incorrect Duration of Residence Values in 2014 (census.gov)