Perception of Time
One issue relevant to performing time-based prospective memory tasks is the ability to accurately judge the passing of time. There has been some interesting research on this topic suggesting that as we get older, we perceive time as moving faster.
There's no dearth of anecdotal evidence here either. As children, a simple afternoon just barely creep along and the school year lasts an eternity. Ask an older adult how quickly time passes and you'll hear just the opposite. There are various explantions (not necessarily mutually exclusive). The attentional explanation is that the amount and type of information we are processing affect time perception (e.g. "time flies when you're busy"), but there are also more psychosocial explanations: as we get older life gets more repetitive, and it is variety and change that leads to the perception that time moves more slowly (i.e., what happens in early life). Check out the links below and let's hear your ideas and input.
The following is a good intro into perceptions of time:
http://ourworld.compuserve.com/homepages/jmkenney/
This book chapter provides some evidence for attentional explanations:
http://www.worldscibooks.com/lifesci/etextbook/5937/5937_chap1.pdf