Problems with time

Time is very tightly tied to geographical position and the date on the calendar. The international standard way of tracking a common time is using Coordinated Universal Time (UTC). UTC is geographically tied to 0⁰ longitude, which passes through Greenwich, England, in the UK. Although it is tied to the location, it is actually not the same as Greenwich Mean Time (GMT). GMT is a time zone, while UTC is a time standard. UTC does not observe Daylight Savings Time (DST):

When data used for analytics is recorded at headquarters or a manufacturing plant, everything happens at the same place and time zone. IoT devices are spread out across the globe. Events that happen at the absolute same time do not happen at the same local time. How time is recorded affects the integrity of the resulting analytics.

When IoT devices communicate sensor data, time may be captured using the local time. It can dramatically affect analytics results if it is not clear whether local time or UTC was recorded. For example, consider an analyst working at a company that makes parking spot occupancy detection sensors. She is tasked with creating predictive models to estimate future parking lot fill rates. The time of day is likely to be a very predictive data point. It makes a big difference to her on how this time is recorded. Even determining if it is night or day at the sensor location will be difficult.

This may not be apparent to the engineer creating the device. His task is to design a device that determines if the spot is open or not. He may not appreciate the importance of writing code that captures a time value that can be aggregated across multiple time zones and locations.

There can also be issues with clock synchronization. Devices set their internal clock to be in sync with the time standard being used. If it is local time, it could be using the wrong time zone due to a configuration error. It could also get out of sync due to a communication problem with the time standard source.

If local time is being used, daylight savings time can cause problems. How will the events that happen between 1 a.m. and 2 a.m. on the day autumn daylight savings is adjusted be recorded since that hour happens twice? Laws that determine which days mark daylight savings time can change, as they did in Turkey when DST was scrapped in September 2016. If the device is locked into a set date range at the time of manufacture, the time would be incorrect for several days out of the year after the DST dates change.

How daylight savings time changes is different from country to country. In the United States, daylight savings time is changed at 02:00 local time in each time zone. In the European Union, it is coordinated so that all EU countries change at 01:00 GMT for all time zones at once. This keeps time zones always an hour apart at the expense of it changing at different local times for each time zone.

When time is recorded for an event, such as a parking spot being vacated, it is essential for analytics that the time is as close to the actual occurrence as possible. In practice, though, the time available for analytics can be the time the event occurred, the time the IoT device sent the data, the time the data was received, or the time the data was added to your data warehouse.