Runescape's farming cycles run based on GMT time (not vastly important for herb runs) and the day starts off at 00:00. What significance is this? Well... because of how farm cycles work!
Herbs run on a twenty minute cycle. They take four cycles to fully grow, assuming they do not get diseased (which stops their growth, and then they die after 2-3 cycles of being diseased...) How does this tie in? Every cycle occurs 1/3rd of an hour, 2/3rds of an hour, or on the hour.
3:00, 3:20, 3:40, 4:00, 4:20, 4:40, etc.
It won't make a significant difference if you don't do multiple herb runs a day, or don't try to be on time. However knowing to wait a few minutes to ensure all herbs on are one cycle can save you 20 minutes of waiting on the last of your herbs to grow, which fellow farmers know is very frustrating. It also lets you keep your farm runs closer to on-time, because you can see when you planted your last herb, let's say at 3:16. The first cycle would be at 3:20, 2nd at 3:40, 3rd at 4:00, and they would be done at 4:20. But what if you planted your first herb at 3:18 and your last herb at 3:22? Now instead of waiting until 4:20, you're waiting until 4:40 for your herbs to be finished! 3:40 (1), 4:00 (2), 4:20 (3), 4:40 (4).
Using a timer is not suggested because of how the farm cycles work. So don't tell me "why not put a timer on for 80 minutes?". This can potentially make you 20 minutes late for your farm run! That's why.
Amulet of Farming and Disease
As far as the amulet of farming goes... unless you're feeling lazy, you should no longer need it to know when your herbs will be done. Look at the clock, round to the nearest 20/40/hour, count up 4 cycles. Your herbs will be ready then. This means its only good for preventing death by warning you of disease.
Disease however stops the cycle until you cure the plant (or it dies..), so if you find a plant has been diseased and you cure it, unless you know the visual cycles (sizes for each cycle) I suggest putting your farm run off for 2 cycles. Disease can be rather irritating as it makes the entire run be delayed 1-2 cycles.
Logging out and game ticks
I personally have yet to experience this, but I read on RSWiki that logging out can reset your current farm cycle. That or I didn't understand the passage. Therefore I suggest if you're getting off at 3:18 and you have some herbs growing. Wait til a few seconds after 3:20 to log off to ensure your cycle doesn't start over if you are planning on getting back on in the next 20 minutes. Otherwise it doesn't really matter.
Because of lag between you and the server and how game ticks work, the times are not exact. Therefore I suggest giving yourself a 1-2 minute breathing room. I start my farm runs 8 minutes until the next cycle and finish with 2-3 minutes to spare.
Trees, bushes and all that awesome stuff
All of these things run on seperate timers that happen at different times in GMT. I don't find it significant since most people only do 1 tree run a day and people who harvest bushes and stuff do it with/after their herb runs. If you're curious for whatever reason you can check out RSWiki (Google; Rswiki Farm Seed Times)
It has come to my attention the only notable search result for being told to "Google RSwiki Farm Seed Times" is this page...
Fruit Trees run on 6 160 min cycles for a total of 960 mins.
Calquat Trees run on 8 160 min cycles for a total of 1280 mins.
21 hours (roughly)
I'll get bushes/normal trees if asked. OTL
Small personal note:
This has held true for me for as long as I can remember, my data matches that of RSWiki, and my farm runs are now done hourly instead of every 80-90 minutes. Meaning my farm runs are on time more often, this means I can get more farm runs done. More farm runs is more experience, and more profit. I don't have to wait around on my herbs to be finished or go check to see if they are finished yet. I know when they are finished. I no longer need to wear my amulet of farming (unless I want to cure it from disease). I'm glad I finally confirmed my data.
Edited by I am Nyu, 30 July 2011 - 12:19 AM.