Complex sentence: Although Ottoman incursions into Bosnia starter before 1382, since Skopsko Krajište was established in 1392 and the earliest year Bosansko Krajište was connected to was 1392 I wrote 1392 as year of its start.
Split into simpler sentences: Although Ottoman incursions into Bosnia started before the Skopsko Krajište was established in 1392 it was the earliest year Bosansko Krajište could be possibly connected to. That is why I wrote 1392 as year of its start.