The RuneApps farming timer allows you to accurately predict when your crops will be fully grown. All crops in Runescape grow in so called growth ticks, it is possible to predict within 5 minutes when these ticks happen. To start using the timer simply click the add timer button and select the patch type and crop type. A progress bar will be shown along with marks for when the growth ticks will happen. The first growth tick can happen right after you plant a crop, this is why the bar on the timer is often partially filled after you set it.
Keep in mind that growth ticks don't occur when the crop is diseased. If you paid a farmer to protect it, it will instantly cured, however a growth tick will still be lost.
It is also possible to lose a growth tick it you log in or out close before the tick.