ABC Tools

Meeting Planner, Explained

After reading this you will be able to find the hours that fall inside everyone's working day across several time zones, read the overlap grid at a glance, and avoid the daylight saving traps that quietly shift the answer.

What the meeting planner does

Pick a date and a set of cities. The tool lines up a full 24-hour day for each city and colors every hour by how comfortable it is to meet locally. Green hours are normal working time (08:00 to 18:00), yellow hours are the fringe (06:00 to 08:00 and 18:00 to 22:00), and the rest is night. Scan down any column and the good slots are the ones where no city is dark.

Here is the hook. Suppose you have people in New York, London and Tokyo. New York and London share most of an afternoon. London and Tokyo share a sliver of the London morning. All three together share almost nothing during normal hours: when Tokyo starts work at 08:00, London is still at 23:00 the night before and New York is at 18:00. The grid makes that squeeze visible so you stop guessing.

When to use it, and when not

Use the planner when you have two or more people in different time zones and you want a recurring or one-off meeting time that respects everyone's day. It answers the question "which hour hurts the least?"

Do not use it to convert a single known time from one zone to another. If you already agreed on "15:00 in Berlin" and just need to know what that is in Denver, that is a one-line conversion. Reach for the Time Zone Converter instead. The planner is for the harder problem of choosing the time in the first place.

The grid is anchored to a specific calendar date, not "today in general." That matters because time zone offsets are not fixed: they change on daylight saving transition dates. A slot that works on 15 January may not work on 15 July.

The offset math behind the grid

Every local clock is described by an offset from Coordinated Universal Time (UTC). London in winter is UTC+0, New York in winter is UTC-5, Tokyo is UTC+9 all year. To compare two cities you convert both to UTC, then back.

t_{\text{local}} = t_{\text{UTC}} + \text{offset}

Here t_{\text{local}} is the wall-clock time in a city, t_{\text{UTC}} is the same instant expressed in UTC, and \text{offset} is that city's distance from UTC in hours on the chosen date. To turn one city's clock into another's, subtract one offset and add the other:

t_B = t_A - \text{offset}_A + \text{offset}_B

t_A and t_B are the wall-clock times in cities A and B for the same instant. The difference that matters is \text{offset}_B - \text{offset}_A. For New York (-5) and Tokyo (+9) that gap is 9 - (-5) = 14 hours. When New York reads 09:00, Tokyo reads 23:00 the same day. That 14-hour gap is why the two cities barely share daylight.

Scoring an hour: the overlap idea

The planner does not just show clocks. It grades each hour. Give every local hour a comfort score: 2 for working time, 1 for the fringe, 0 for night. For a given UTC hour, add the scores across all cities. The best meeting hour is the one with the highest total.

S(h) = \sum_{i=1}^{n} c_i(h)

S(h) is the total comfort for the UTC hour h, n is the number of cities, and c_i(h) is city i's comfort at that hour (2, 1 or 0). The maximum possible score is 2n. With three cities the ceiling is 6. If the best hour only reaches 4, someone is stuck in the fringe no matter what you pick.

With cities 5 hours apart, a full 08:00 to 18:00 working window overlaps for 6 hours; at 8 hours apart the shared working window shrinks to 3 hours; at 11 or more hours apart the working windows stop touching and the best you can reach is a fringe-to-fringe pairing.

Worked example: New York, London, Tokyo on the demo date

Take the three default cities in winter, with offsets New York -5, London 0, Tokyo +9. Walk one candidate UTC hour through the math.

  1. Pick 13:00 UTC. New York is 13 - 5 = 08{:}00, London is 13 + 0 = 13{:}00, Tokyo is 13 + 9 = 22{:}00.
  2. Score each: New York 08:00 is working (2), London 13:00 is working (2), Tokyo 22:00 is the very end of the fringe (1).
  3. Total S = 2 + 2 + 1 = 5 out of a ceiling of 6.
  4. Try 14:00 UTC instead: New York 09:00 (2), London 14:00 (2), Tokyo 23:00, which is night (0). Total drops to 4.

So 13:00 UTC is the sweet spot: two people fully in work hours and one just before bed. Push an hour later and Tokyo goes dark, costing you a point. Push an hour earlier to 12:00 UTC and New York slips to 07:00 (fringe), also a 5 but now the fringe burden moves from Tokyo to New York. The planner lets you choose who absorbs the discomfort.

The score peaks at 5 around 12:00 to 13:00 UTC, then falls as Tokyo moves into night. No hour reaches 6, which tells you a fully comfortable slot for all three does not exist on this date.

Reading and interpreting the grid

Three habits make the grid readable. First, scan columns, not rows: a column is one instant seen in every city at once, and you are shopping for columns with no grey. Second, count the yellows in your best column: a column of all green is rare across wide spreads, so most real answers involve one or two people in the fringe. Third, decide the fairness rule before you look. If the same person is always the one at 22:00, rotate the meeting time across weeks so the burden moves.

Interpret a top score below 2n as a signal, not a failure. A ceiling of 6 with a best of 5 means one person gives up an hour of comfort. A best of 4 across three cities usually means two people are in the fringe or one is close to night, which is a candidate for splitting the group or recording the session instead.

Common mistakes

The biggest error is planning against the wrong date. If you build a grid for a Tuesday in March but the meeting is in November, a daylight saving shift can move a city's offset by an hour and turn your 5-score slot into a 3. Always set the date to the actual meeting date.

A second mistake is forgetting that daylight saving does not start on the same day everywhere. Europe and the United States switch on different weekends, so for two or three weeks each spring and autumn the gap between, say, London and New York is 4 hours instead of the usual 5. During that window your normal slot drifts by an hour.

A third mistake is treating the fringe as free. An hour at 07:00 or 21:00 is workable once, but a weekly meeting parked there wears people down. Read yellow as a cost you are choosing to pay, and share it around.

A fourth mistake is mixing up date lines. When New York is at 08:00 on Monday, Tokyo is already at 22:00 on the same Monday, but at some UTC hours Tokyo has rolled into Tuesday while New York is still on Monday. The planner tracks the calendar day per city; trust its labels rather than your mental arithmetic.

Related tools

Once the planner has told you the winning slot, you often need to state that instant precisely for one person's calendar. The Time Zone Converter takes a single date and time in any IANA zone and gives the exact equivalent in another, including the correct offset for that date. Use the planner to choose the hour, then the converter to confirm the wording of the invite.

Frequently asked questions

What counts as working hours in the grid?

Working hours are 08:00 to 18:00 local time, scored 2. The fringe is 06:00 to 08:00 and 18:00 to 22:00, scored 1. Everything else is night, scored 0. These are fixed bands, so a 07:30 start always reads as fringe regardless of the city.

Why does the best slot change between January and July?

Because offsets change with daylight saving time. London is UTC+0 in January and UTC+1 in July, while Tokyo stays at UTC+9 all year. That one-hour shift moves the whole overlap, so a slot that scored 5 in winter can score 4 or 6 in summer for the same cities.

How many cities can I compare at once?

You can add as many as you need, but the comfort ceiling is 2n and it gets harder to reach as n grows. Beyond four or five widely spread cities, a single slot that keeps everyone out of night is often impossible, and splitting into two meetings works better.

Does the tool send my cities to a server?

No. Everything runs locally in your browser. The date, the cities and the grid are computed on your device and nothing is transmitted.

What if the best score still puts someone at night?

Then no shared awake hour exists for that set of cities on that date. Your realistic options are to rotate who takes the awkward time each week, to record the meeting for the person in the dark, or to run two smaller meetings grouped by region.

Try it yourself in the tool

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