I'm on the phone right now but let's see if I can explain.
In the function node:
The data is an array where every element has timestamp.
You'll need to read that array from context.
var Yourarray=context.get("Yourarray") || 
Then do your stuff with that array
And may be add new entry to array.
Before storing the array back to context filter out all elements where timestamp is older than 48 hours. Note that all calculations are done in milliseconds.
var time = Yourarray[Yourarray.length - 1].timestamp - 172800000
Yourarray = Yourarray.filter(el => el.timestamp > time);
And now the array is ready to be stored back into context.