@Lokal_Profil Would it be OK to upload them with the value 1927-1931 and a maintanence category e.g. Media_contributed_by_SMVK_with_estimated_date

You could use Template:Between in the date field: 1927-1931 → {{Between|1927|1931}}.

There's also {{other date|year unknown}} that can also be used in the date field.

Then as for the category, do you already categorize the images by year? If so, adding a category with undetermined year does make sense.

Nice one with the Template:Between.
OK, though it's tricky since most of the images with no date is obviously taken on Cyprus and thus must have been between 1927-1931 during the travels there.

No, no category with year of image. I was thinking this was a way of documenting that the metadata, in that case, have been altered.

In T162355#3160150, @Mattias_Ostmar-WMSE wrote:

OK, though it's tricky since most of the images with no date is obviously taken on Cyprus and thus must have been between 1927-1931 during the travels there.

So that's a comfortable situation for us, at least! It's always good to have some sort of estimate, and in this case it's just a couple years. So not having an exact date is not the end of the world.

I would probably skip the maintenance category and just use {{Between|1927|1931}} or {{other_date|-|1927|1931}} (did I remember that right?)

PR: https://github.com/mattiasostmar/SMVK-Cypern_2017-01/pull/33

Merged: https://github.com/mattiasostmar/SMVK-Cypern_2017-01/commit/a36d33678e31f0b8803d8d9df95038a70fa1eb33