I know I've seen it in the books somewhere but I can't remember where or what the date was. So, what's the starting date for deathwatch please and where is it recorded?

817.M41

What AK said, and here's the location in a book for reference (probably others somewhere)

Deathwatch Honour the Chapter, P27
Blood Ravens Chronology sidebar:

(Some stuff about the events of Relic Dawn of War series)...it is vital that they consider the information presented throughout the Deathwatch game line is set in the year 817M41.

Thank you so much. I just found it a few moments ago.