#1 rise and set times change periodically

[Scott Johnson]

On 30 September near 24:00 localtime, I noticed that
every two minutes rise and set times would shift between:
yes: 10:33 20:29
tod: 11:50 21:13
tom: 13:03 22:05

and:
yes: 11:34 21:29
tod: 12:50 22:12
tom: 14:03 23:05

On 1 October, near 00:00 localtime, I saw similar
behavior. The interval is longer, about 7 to 10
minutes, and the times shown are:
yes: 13:50 23:13
tod: 15:03 no set
tom: 16:07 00:05

and:
yes: 12:50 22:12
tod: 14:03 23:05
tom: 15:07 no set

The second set of values corrisponds to the second set
for the previous day, but the first set does not.

I am at Latitude +40 and Longitude +111, in time zone
MST7MDT with my hardware clock set to localtime. That
should be enough information to duplicate my settings
for testing.