Recording numbers with dates and units you can trust

A recorded number is only useful if you can still interpret it later. A figure written down without its date, unit or definition may be perfectly clear when you write it, but a few months on it can mean several different things, and the difference matters when you compare it with other entries. The fixes are small: a consistent date format, a named unit, a short definition and a note whenever conditions were unusual.

The details that make a log trustworthy are separate from the question of which measures to track. They take seconds to add when you record an entry, and they are almost impossible to reconstruct afterward.

Write dates in one unambiguous format

Dates cause more confusion than they should. A date written as 03/04 means the third of April in some countries and the fourth of March in others. Choosing one format and using it throughout removes that ambiguity. The ISO 8601 standard writes the year first, then the month, then the day, separated by hyphens, and it sorts correctly in a spreadsheet or a file listing. Whichever format you choose, record the date on every entry rather than relying on the position of a row.

Record the time as well when it could affect the number. A reading taken at seven in the morning and another taken at nine in the evening may differ for reasons unrelated to what you measured. If time of day could matter, add a time column. When an entry covers a period, such as a month's electricity use, write the start and end dates rather than only the day you wrote the figure down.

Keep the date of the event separate from the date you wrote it down when the two differ. A fuel purchase made on a Friday but entered on Monday should carry Friday's date, and the entry date can sit in a second column if it is useful. When you travel across time zones and record a time of day, add the zone or note that the time is local, because a reading taken at the same clock time in another zone may reflect a different part of the day.

State the unit every time

A bare number is incomplete. The same value can mean very different things depending on the unit. A gallon in the United States is about 3.79 liters, while the imperial gallon used in the United Kingdom is about 4.55 liters, so a fuel figure recorded in gallons is meaningless without knowing which one was meant. A temperature of 20 is mild in Celsius and well below freezing in Fahrenheit. A distance written as 10 could be kilometers or miles.

Put the unit in the column header rather than in each cell. That keeps the cells clean and makes the unit visible every time you look at the column. If a column mixes units because you changed method partway through, convert the older entries and note the conversion in the log. Keep one unit across the whole log whenever possible, since each conversion introduces rounding and another chance for error.

Decide how many decimal places to keep and stick to them. A column that mixes four-decimal and whole-number entries is hard to scan, and inconsistent rounding makes averages and comparisons awkward. Record the value as read or measured, and round only when you present the figure. Derived measures need the same care. A figure such as miles per gallon, or cost per month, depends on the units of the numbers it came from, so write the formula in the column note. Then anyone reading the log, including you later, can see how the figure was produced and check the arithmetic if a value looks wrong.

Add a definition and a note for unusual conditions

Write a one-line definition for each measure, saying what is included and what is excluded. A "workout" could be a gym session, a run or any stretch of time spent moving hard enough to breathe faster, so the definition should say which of those count and whether a brisk walk does. This definition is what makes an entry from six months ago comparable with one from later on, even when you have forgotten the context that surrounded it.

Add a short note whenever conditions were unusual: a different route, a new device, an unusually cold week, a change in how you counted, or simply a day you felt the reading was off. Keep the note brief and factual. "Meter read from the outside wall, not the basement" is more useful than a long description of the afternoon. Notes are the cheapest insurance a log can have, because they explain the entries that would otherwise look like mistakes when you review the record months later.

Definitions also change over time, and that is acceptable when the change is recorded. If you decide to count a new kind of expense from a certain date, write the change in the log on that date, and keep the old and new definitions side by side for a short overlap if you can. Without that record, a sudden step in the numbers looks like a real change in behavior when it is only a change in what was counted.

A final check is worth making every few months. Pick three older entries at random and see whether you could still explain each one from its date, unit and note alone. If you cannot, the log needs a clearer habit at the point of recording, not a different set of measures.