Farmer villagers not harvesting or planting crops

We have confirmed that farmers will not harvest until they already have something in their inventory that they can plant, as reported by this user comment.

Additional reasons they won't harvest or plant, which are not bugs, include:

• The mob griefing gamerule is toggled off. Like endermen picking up grass, when farmers pick crops it's considered griefing.
• It's not their working hours. Farmers will only harvest and sow during their working hours (that is, times 00000–07999 and 10000–10999 as given by /time query daytime). This excludes their scheduled times for mingling, going home after work, and sleeping.
• It's raining. Villagers don't work when it's raining, instead they look for shelter (a solid block they can stand under). For farmers, this means they won't sow or harvest even if the crops are under cover with them. (If they're unable to find shelter, farmers will continue to harvest. We don't know whether this is intended behavior or a bug.)

Please limit comments to new information about harvesting and planting. You may search for/create separate tickets for other issues with villager behavior.