Last night, I was just checking our email alerts before I go to sleep, I noticed that there are high number of errors in one of our backend services. Weirdly enough, it started at exactly 12am.

When I checked the logs, I noticed that the issue was caused by one of our methods that deals with time. It puzzled me for a while since that feature was running smoothly for almost a week now. We also had tests that covers it. Upon further investigation, I realized that…

Sundays are different in Scala.

To provide a bit of context, this part of our backend code that deal with dates was migrated from C# to Scala. This backend code also uses a config file that have dates represented as numbers. Sunday is 0, Monday is 1… Saturday is 7. We did it this way to follow how C# deals with days of week.

Our mistake is that we didn’t know that day of week is represented differently in different programming languages.

• For C#, Sunday is 0, Monday is 1, and Saturday is 6. Here’s the documentation.
• For Java (hence Scala), Sunday is 7, Monday is 1, and Saturday is 6. Here’s the documentation. It’s not 0 based, and week starts at Mondays.
• I also checked for Python and it’s also different. The default is that Monday is 0, Saturday is 5, and Sunday is 6.

In these three programming languages, Sundays are represented in three different numbers.

Moral of the story: We need to write better test cases.

I don’t have much to tell anymore, _{but I feel that weeks should start on Sundays, right?}