Calendar systems and their role in patent documentation

Part III: Buddhist calendar and Hijri years

This is the final part in our series on non-Gregorian calendar systems and the challenges they present for the patent searcher.

We've already looked at Japanese imperial years and Taiwanese Republican years.  In this edition we take a look at further examples of timekeeping - the Buddhist calendar and Hijri years. You encounter these calendar systems when searching patent information from South-east Asia and the Arab world.  

Thai Buddhist calendar

The Buddhist calendar is widely used in South-east Asian countries which follow the Theravada school of Buddhism, in particular Thailand, Cambodia, Laos, Myanmar and Sri Lanka. The day on which Siddharta Gautama - the historic Buddha - reached the parinirvāṇa after passing away is taken to mark the epochal year zero, the start of the calendar. However, as the exact date of the Buddha's parinirvāṇa is uncertain, when exactly the calendar starts differs slightly from one country to another. In Thailand, Cambodia and Laos the Buddhist year zero corresponds to the Gregorian year 543 BC, and in Sri Lanka and Myanmar to 544 BC.

Apart from the starting date, the Buddhist calendar is identical to the Western Gregorian calendar - you can calculate the Buddhist year in Thailand by simply adding 543 to the Gregorian year. For example, the Western year 2000 corresponds to the Thai Buddhist year 2543, and 2022 to the Thai Buddhist year 2565.

Why is this relevant to patent searchers? As Fig. 1 shows, the Buddhist Calendar also plays an important role in patent documentation. All references to dates in the Thai patent document are shown in Buddhist years only. You can convert these to Western Gregorian years by subtracting 543.

Calendar systems fig 1

 Fig. 1 First page of a granted Thai patent (click to enlarge)

Calendar systems fig 2
Fig. 2: Extract from DIP's IP Information Search Service (click to enlarge)

Unlike in the original documents, public search tools, such as the IP Information Search Service provided by the Thai Department of Intellectual Property (DIP), usually display Gregorian years (Fig. 2). Unfortunately, Thai data is not yet stored in EPO databases and is therefore not accessible in Espacenet. It is therefore recommended to use the DIP's public search tool as it provides access not only to bibliographic data, but also to legal event data, original documents and some file wrapper information.

Hijri Calendar

The Hijri calendar is used in almost all Islamic countries around the world, usually in parallel with the Western Gregorian one. It dates back to the year 622 AD when Muhammad and his followers migrated from Mecca to Medina - an event referred to as the "Hijrah". The first day of the Hijrah (26 July 622 AD) is taken to mark the first day of year one of the Hijri Calendar.

It is not possible to directly convert a Hijri year into a Gregorian year. As Hijri years are based on the moon's phases, they are slightly shorter and contain only 354 or 355 days. The current Hijri year is 1444 and runs from 30 July 2022 to 18 July 2023.

Again, how is this relevant to you as a patent searcher? In some jurisdictions, patent documents use dates with Hijri years. However, unlike with Buddhist years in Thailand, patent documents do not usually use dates with just Hijri years. They usually also contain Western Gregorian years. In the example from Saudi Arabia below (Fig. 3), you will see that while the filing date (INID code 22) and publication date (INID code 45) of the granted patent both use Hijri and Gregorian years, the dates related to cited prior art documents (INID code 56) even only use just Gregorian years. Espacenet contains bibliographic data on Saudi Arabian pending applications and granted patents as well as original documents.

Calendar systems fig 3
Fig. 3: First page of a granted patent from Saudi Arabia (click to enlarge)

These examples show how different calendar systems can impact your searches. Even though most jurisdictions around the globe stick to the Western Gregorian calendar, you can still find other timekeeping methods in patent documentation. If you come across unusual date formats in patent documents or need any further help in your daily work with patent information from jurisdictions outside Europe, feel free to contact the EPO's Worldwide IP team at

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