It's okay to use the start event time for events like nursing that have a small duration but it makes for a weird experience for events like sleeping where if the baby slept 4 hours and woke up right now it'll show as "Last time 4 hours ago" in the add screen.
It seems like the information inside of the add button is not reloading either when letting the app sleep in the background or even when completely closing and re-opening it.