It's been more than 6 weeks since I powered on the new voltage reference. It has been running continuously, except for 2 relatively short power outages. Time to find out what sort of drift is happening. Here's the data:

Temperature = 64F (~18C), HighZ input, 100plc

Reading                                 Scale             Other                                   Error             Drift

99.9913mV - 99.993mV        100mV           Null, Avg 20 samples         ~-8uV            +1.4uV (14ppm)

1.0000829V                           1V                  Avg 20 samples                 +83uV           -0.4uV (-0.4ppm)

10.000070V                           10V               Avg 20 samples                   70uV            +83uV (8.3ppm)

I'm satisfied with these results. The biggest surprise is the 10V reading drifting nearly 10ppm, but it doesn't require any adjustment since it is within 100uV of the target. I'll just record these numbers on the box and use them to compare readings in the future.

Note that the 100mV reading is a range. I left the meter run continuously over several days and made multiple readings. Every time I took a null measurement and replaced the probes it would be a slightly different reading. I'm going to chalk that up to thermoelectric noise and move on.