Thanks for the response. I am doing as you indicated. However, my problem seems to be with the iMac. When I started to perform the same task on my PC, I was able to successfully change the capture time as you have instructed. Thanks.
What four-digit year are you trying to enter on the iMac? Is the year set correctly on the iMac itself?
I'm not quite sure if this is the same problem, but I am in a similar situation, only with .tif files.
Trying to reset the date to the actual slide capture date. Month and day will change as expected, but when I try to set the year to 1999, that won't display. Instead, when switching through the years via the down arrow key, we descend 2001, 2000, 199, 1998,...
So I hit cancel, then tried again, this time just selecting "2015" and then began typing. I entered 1 and that appeared. I entered 9 and 1 disappeared, but the 9 appeared in its place. If I use the left arrow to leave the year field, then the right arrow to return, I may type in 199, but not 1999.
Entering the year on Mac LR can be fussy sometimes. I have to type the four-digit year quickly (and I'm a fast touch typist). Try copying "1999" from another program and pasting it into the field.
Also, please add your vote and details of your issue to this bug report in the official Adobe feedback forum: Lightroom: Unable to set capture date to 1966 (Mac).Probably best to continue the discussion in that thread, so everyone benefits.
I updated today (now in LR 6.3) and discovered that this appears to have been fixed. Woot!