Problem: Currently it's hard to use duration property, because displayed value is not user friendly. It's hard to add source, when I see, that Adele's Rolling in the Deep is 228 second long -- it should display 3 minutes 48 seconds instead. It also hard to insert new data, because you need to recalculate value.
Who would benefit: Wikidata readers and editors, especially in cultural area (duration of movies, albums, songs...).
Proposed solution: There should be possibility to enter value via Wikidata interface in HH:mm:ss or mm:ss format. It could be recalculated to base unit (seconds) on front-end side and saved to database. Same situation with displaying values.