Date: Thu, 25
There is a simple explanation for the date discrepancies: Everyone is right! It happened BOTH on June 29 and 30, 1954, depending on the time zone. Using local time, Atlantic Standard Time, it was the 29th but using GMT it was the 30th. See technical discussion below.
This is a Condon Report unexplained sighting (see pp. 139-140, listed as unexplained pp. 173, 961). It was described by Condon Committee physicist Gordon Thayer as an "almost certainly natural phenomenon, which is so rare that it apparently has never been reported before or since." This is a classic phrase often quoted by critics of the Condon Report as indicative of the evasion of the scientific issues involved in the UFO problem.
Technical discussion to follow (most readers may simply skip this):
Although Timothy Good's treatment doesn't give the time of the sighting it does indicate it was about 3 hours after a series of holding pattern delays, etc., up to about 2200 GMT on June 29, which would put the UFO sighting at about 0100 GMT on June 30. (Though 3 hours seems too long to travel at 230-240 knots the approximately 500 nmi from Cape Cod vicinity to Seven Islands, Quebec, it should be more like 2 hours and this is supported by the sunset calculations below.) Either way, local time would have been 2100-2200 Atlantic Standard Time on the 29th but GMT would have been at or after midnight the 30th.
The Condon Report gives the time as 2105 to 2127 local standard time which would be 0105 to 0127 GMT on the 30th but this seems too late because the sun would have set but according to the witnesses was still visible low on the horizon, and the plane landed at Goose Bay at 0145 GMT three hundred miles away. (McDonald gives the sighting duration as 18 minutes rather than 22 minutes and I can't resolve the discrepancy.)
At the airliner's speed backtracking the 300 nmi to Goose Bay would put the UFO sighting at just after midnight, around 0000-0015 GMT. If the Condon Committee converted the time zone incorrectly but had the exact minutes of the sighting correctly from some source then the sighting perhaps was from 0005 to 0027 GMT.
The sun would have been about 7 degrees above the apparent visible horizon (correcting for aircraft height and refraction) at true azimuth 302 degrees or roughly WNW at 0005 GMT near Seven Islands, about 50 N, 67 W (this location given seems too far west for a heading to Goose Bay from Cape Cod, and seems should be closer to about 63 W, which would drop the apparent elevation of the sun to about 5 degs at 305 degs azimuth). According to McDonald who interviewed airline Capt James Howard, the UFO was about 5 degrees to the left of the sun, which would put the azimuth of the UFO at about 297 degrees true, initially.
As the airliner flew northeast into Goose Bay over the next twenty minutes or so the sun would have continued to drop towards sunset. At the presumed position when last sighted at 0027 GMT, the sun would have been about 3 degrees above the apparent horizon at true azimuth 309 degrees (presumed position about 51 N, 64 W; at what seems to be the more correct flight path, at 52 N, 62.5 W, the sun's position would have been about the same elevation but at 310 degs azimuth).
However if we used the three-hour time from Timothy Good or the apparent one-hour time zone error from the Condon Report the sun would have been below the horizon at all of these times and locations. Hence the sun's position supports a time of occurrence just after midnight GMT, rather than an hour later.