informal and common; describes both people and events
用法筆記
Commonly describes a person's habit or character rather than a single late occurrence. For one isolated event, 'late' is more natural: 'I'm sorry I'm late' (not *unpunctual).
常見錯誤
I'm sorry I'm unpunctual.' (for a single missed start time)
I'm sorry I'm late.
unpunctual describes a regular behaviour pattern, not one occasion.