General Visiter 6 month Rule

Hello,
Thanks in advance for stopping by and answering this for me.

I have family and friends in the UK. So in 2013 I visited twice, once in summer, and over the christmas holidays.

I am planning to visit for a longer period of time, 5 months.

I have read you can visit the UK up to 6 months in a year.

My question is my previous 2 visits were in 2013.

Will my new trip include those previous trips (time)?
Or because it is now 2014 I will be allowed up to the 6 months?

Thanks!

See also Visitors Guidance and VAT1.5 Frequency and duration of visits.