I'm scanning the entire family archives, and obviously there's no specific capture times for a lot of the old photos. For some photos I can only guess the year they were taken, and I want to be able to sort photos based on the year they were taken. When changing Capture Time for a photo, however, I also have to add the month and day, with makes this feature useless.
Wouldn't it be nice if we had 00/00/Year as a possibility? Or are there other ways?
You can also add the four digit year as a keyword. Then use the Library Text filter with Contains Words and then enter the four digit year. The actual capture date of the scan file may be useful at a later date. You should also determine what other metadata you would like to add and how. Some good books on the subject by Peter Krogh at the below link. The book Organzing Your Photos with Lightroom 5 is still relevent with LrC 11.
As others have suggested, using a dummy mth+day like 01/01 is the best and to be frank is not much different than 00/00 and still respects the date format. What you are asking for is that LrC allow an invalid date to be entered.
I understand your pain, I've been scanning old negatives and slides. If I know it's the fall, I'll set it to yy/10/01. If it's the spring, I'll set it yy/03/01. If I don't know, I set on the convention 1977/07/07 just so visually I'll know I have no idea.
I wondering if you remember you can select photos just by year in the library filter and if that would be good enough.
To build on the other recommendations, for my archival scans I use the following:
year-01-01 00:00:ss -- only the year is known
year-mm-01 00:00:ss -- only the year and month are known
year-mm-dd 00:00:ss -- only the year, month, and day are known
I assign increasing values to "ss" for all the scans from a given roll, so that their order on the roll will be preserved when displaying them in LR with Sort: Capture Time.
I never use "00:00:00" since in years past some apps got confused comparing times of "00:00:00". There are the obvious amgiuities -- is year-mm-01 00:00:01 have the known day-of-month of 01 and unknown time, or an unknown day-of-month? I live with the ambiguities, rather than adding additional metadata to the scans.
The XMP metadata standard formally allows for date/times with unknown month, day, time, or time zones:
But the older EXIF standard does not. While there is an industry standard that specifies XMP fields should be used in preference to EXIF fields when both are present, most apps and services don't implement that standard.
At least in versions of Photoshop Elements up through 9, it allows for partially unknown date/times, and I used that for my scans. But it's implementation was bug-ridden (as was most of PSE's Organizer). When I migrated from PSE to LR, I adopted the convention described above.