When is a leap year not a leap year? Don't ask Microsoft Excel

February 27, 2021

Dates and times always have the potential to cause trouble in code; back in my apprenticeship, my teacher drove this point home by sharing a great article called Falsehoods programmers believe about time. Since then I’ve been wary of working with data that relies heavily on dates. (If you cannot tell, this paragraph’s job is to supply foreshadowing).

In a recent project at work, I’ve been collecting Covid-19 data from various government sources and consolidating them into one standardised collection. The best scenario is that a country’s government has a well-documented API. Typically countries seem to publish daily PDFs or CSVs that keep having data appended to the bottom. The worst-case was a certain health ministry’s website that would include yesterdays data in the homepage’s HTML and then update the HTML the next day with the previous day’s stats.

An interesting date problem I recently encountered was in the data from Belgium. Belgium publishes its Covid-19 data in an Excel file with separate pages for cases, deaths and tests data, and when parsing the file using some JavaScript I found that one of these pages was returning dates as a number instead of a string. This is because Excel uses the 1900 date system where dates are expressed, behind the scenes, as the number of days after the start of 1900. Ok then. I started writing a function to convert the number into a date string that matched the dates from the other pages.

const convertExcelTimeToTimestamp = (dataValue) => {
    const DAYS_OFFSET = 25569; //days between start of 1900 and 1970
    const MILLISECONDS_IN_A_DAY = 86400000;
    const unixDate = (dateValue – DAYS_OFFSET) * MILLISECONDS_IN_A_DAY;
    const timestamp =  moment(unixDate).format('YYYY-MM-DD');
    return timestamp

The function subtracts the number of days between the start of 1900 and 1970 from the input, to get the date in +/- days relative to 01/01/1970. Multiplying this by the number of milliseconds in a day gets a Unix time value that’s usable in JavaScript. To figure out the value of DAYS_OFFSET I initially used an online date difference calculator to get the rough value and then relied on a unit test to check it’s correct. I took the first 1900 date serial number from the Belgian Excel spreadsheet, 43891, and made a test asserting that the function returned the date shown in Excel, “2020-03-01”. Cool, test passes, done.

But then I ended up in a confusing loop when I began adding more unit tests.

The definition of the 1900 date system said that it starts with 1 = 01/01/1900, so I made a unit test to assert this. But that test failed due to an out-by-one error. Confused, I then adjusted the function’s calculations to make that test pass, but this then caused the tests for dates in 2020 to start failing with an out-by-one error instead. This left me scratching my head… why would the function work differently at low and high input values?

Expected: “1900-01-01”
Received: “1899-12-31”

Turns out my error was trusting Excel. For historical reasons, Excel has to support a bug where 1900 is falsely believed to be a leap year. This means that Excel believes there’s an extra day between 01/01/1900 and 01/01/1970, impacting the value of DAYS_OFFSET in my function. It works fine for recent dates, but if I use the function for dates before the fictional leap day 29/02/1900 then the offset value is incorrect and I get an out by one error.

The moral of the story - think about whether your unit tests cover scenarios or edge cases that will occur in your app, and don’t add unneeded unit tests. The project I’m working on was never going to receive dates from the early 1900s and if I hadn’t begun adding unnecessary tests I wouldn’t have stumbled on this confusing error and spent 30 minutes questioning the space-time continuum.