Part One: Strategy · Deciding what to build
Asking how long something will take invites an estimate that grows. Deciding first how much the problem is worth sets a limit the solution has to fit.
"How long will it take?" sounds like the responsible question. The answer is an estimate of a solution nobody has examined yet, and the work found along the way adds to it. Nobody has asked whether the problem deserves that much.
A calendar can be two weeks of work or six months. Which is right depends on how much trouble the lack of one is causing, compared with everything else you could do.
Decide first what the problem is worth: say, three weeks and no more. Count calendar weeks, from starting to having it in people's hands, with any waiting on other people included. Three weeks of work that sits in two other queues takes a season. Then ask what the best version is that fits. The rare cases go. The clever extras go. So does anything that needs another team's queue. What is left is the part that mattered.
Both sides resist. The person who asked wants the whole thing, and the person building does not want to be held to a limit. It protects them both: when the time is up you stop and look, instead of drifting on because so much has been spent.
Customers had asked Basecamp for a calendar for years. A full one would have taken months, and only about a tenth of customers had used the calendar in the earlier version. The team decided the request was worth six weeks and built a two-month grid with a dot on each day that had something scheduled, and a list of events beneath. It was enough for the people who needed it.
Therefore:
Before designing anything, decide how much time the problem is worth, and treat that as a limit, not a guess. Then look for the best version that fits. If none fits, the answer is not yet.
In practice. For the next request, finish this sentence before discussing solutions: "This is worth at most ___ of our time, because ___." If you cannot fill in the second blank, you do not understand the problem yet.
Goes with. The limit shapes the idea while it is ROUGH ENOUGH TO CHANGE (22) and sets the size of THE SMALLEST WHOLE THING (26). Whether the time was well spent, you learn when you CHECK AFTER YOU SHIP (28).
Source: Ryan Singer, Shape Up (2019), which tells the calendar story.