@murrayjames,

1. 当天dàng tiān: 同一天， 就在本天. The same day; that very day.

i.e. 当天的事，当天做完； 你可以当天来回。（the same day）

2. 当天 dāng tiān: 过去的某一个时间. Sometime in the past.

i.e. 他当天就回老家去了.

It's the same usage as 当年

dàng nián: 同一年，就在本年. The same year; that very year.

dāng nián: 过去的某一段时间. In those years or days; that year.

Hope it helps out:)