The further you can move the basis of timing evidence away from the possibility of human error and into the realms of science, the better.
The most common method of external timing is TV - when something happened in a programme, or when a programme began or ended. Remember that when someone notices something of significance whilst watching the TV, they miss a section of the programme that they were watching. Returning to the programme later means they have missed something and have to catch up. This may be the point that they remember best.
Street lights are also reasonably good timers. Their timings need to be checked very soon after the crime, because many work on light meters. Trains and buses passing nearby may also provide a timing - as may private cars at certain times of the day because people sometimes use their cars at times that they will remember - such as going to work, taking the children to school etc.
Weather can be a good timer on occasions - was it raining when someone set out, at what did time did you take the washing in? Reference has already been made to shadows in police photographs, the same principle might also apply in other areas of the investigation. People with good visual memory may remember whether something was in the shadow or not at the time of the instance they are a witness to. And of course there is the "broken watch" syndrome, a favourite of detective stories.
Scientific deduction of the passage of time in non-murder cases is often reduced to blood. Blood dries at a reasonably regular pace and can often be a good guide. It also "haloes" in rain - so bloodspots might indicate when the blood was spilled. Bruises - another aspect of blood - can also be timed. If bruises have been used in the police investigation, you should look for the photographs of the bruises that should have been taken. As everywhere else in the investigation, if this routine type of evidence has not been taken and disclosed, you should wonder why.
Other areas of scientific investigation leading to the precise timing of a crime are rare - there may be a specific instances, but these cannot be generalised. That does not mean however that you should look for such an input, depending on the type of crime you are investigating and the specific events or nature of the crime. Always remember that in preparing an appeal, evidence with a scientific base is better than evidence from witnesses. Such a rare occurrence, for example, would be wet paint - when was it applied, how quickly does it dry - and how easily could the victim have got it onto his/her clothing.