Thanks for steping in, jemidiah, I havn't had a coffee by the time I was replying.

@SLH
Ideally i want a function that can give the value as a function of start value, end value, and time (between 0 and 1)
Your time-value will be anything between 0 and 1? In that case you need to define useable timesteps in order to use the function mentioned by jemidiah.