Climate normals are computed over 30 years. Not sure why that number of years was chosen specifically. Perhaps a recent incentive is to identify climate changes (though it actually would make warming appear less significant when looking at anomalies). But, given that those trends have only gotten interest in the past few decades, the only historical reason I can fathom is to sooner allow more sites to be used. 30 years is a fair number of data points statistically, but for daily normals, it should still have a significant error range. It would seem a 50 or mean year normals length might be more useful at this point, though indeed climate changing might be comparable to the confidence interval.
But regardless of why they chose that value, that should be the period they are reporting.
If you wanted to verify these values, I was able to find pages to get the 1961-1990 normals (use the find command within this file to determine station id), 1971-2000 normals, and of course 1981-2000 normals.
Or a more robust option would be to use NWS' great ClimateNow data portal (select your local NWS office). There you can look at annual data for the entire record length, search for specific values, or compute normals for any period you wish (select your station, then Monthly summarized data, and then the mean at the bottom is your normal over the time period you selected) (can also change to avg temp).
I know these portals can be a bit complex to some, so if you wish to mention the area, I can pull the values and edit the answer. And then you can see just how much the normals have changed :-)