ScatterChasse, from Wikipedia:
Series one
The first series started on 26 September 2010, and explored the lives of the Crawley family and their staff from the day after the sinking of the RMS Titanic in April 1912 to the outbreak of the First World War on 4 August 1914.
So it spans two years 4 months pretty much.
Series two
The series will be composed of eight episodes, running from the Battle of the Somme in 1916 to the Armistice in 1918.
So the second series also spans two years and four months (if you take the start date of the Battle of the Somme). Surely if we've only seen four episode we can only be somewhere in 1917?
For some reason I thought there were 9 episodes in series 1 but there were only 7. As there are 8 in this series and both series cover the same time span, the first series must have gone faster than this one. Doesn't seem like it though.