We stick calendars on the wall or load them up on our phones, but the people of the third millennium BCE used giant rocks, new research suggests.

A new study explains how Stonehenge may have originally been used to keep track of a solar year (aka tropical year) of 365 and a quarter days, which has long been suggested by researchers, but never fully understood.

The new findings are based on a careful analysis of the number and the positioning of the stones that make up the site, as well as comparisons with other ancient calendar systems that might have influenced the builders of Stonehenge.

Studies of Stonehenge as a way of tracking time and seasons stretch back centuries, but until now it's remained unclear exactly how this might have worked.

The new research was built on a previous study revealing that the sarsen stones that make up the bulk of Stonehenge all came from the same source. That means they were likely to have been put up at the same time, and were probably intended to work together.

From that starting point, archaeologist Timothy Darvill, from Bournemouth University in the UK, went on to look at the positioning of the different rings that the monument is comprised of, and how they may have related to a calendar.

Archeologists have long suspected Stonehenge was a calendar of sorts, due to the positioning of the stones and their alignment with the solstices, and the new research adds weight to the interpretation.

"The proposed calendar works in a very straightforward way," says Darvill. "Each of the 30 stones in the Sarsen Circle represents a day within a month, itself divided into three weeks each of 10 days.

"The intercalary month, probably dedicated to the deities of the site, is represented by the five trilithons in the center of the site. The four Station Stones outside the Sarsen Circle provide markers to notch-up until a leap day."

In serving as a solar calendar, the winter and summer solstices could be viewed through the same pairs of stones each year.

This would have acted as a way of checking errors, Darvill suggests. If the Sun were ever in the wrong place on the solstices, then the ancient people of Wiltshire would have known that they'd gone wrong somewhere in calculating the year.

None of the arrangements within Stonehenge seem to match the 12 months that make up a year, the new study notes, but it's possible that some of the missing or moved stones on the site were responsible for keeping track of these. What is clear is that the architecture of Stonehenge has been split into two halves to match the two solstices.

Weeks lasting 10 days may seem unusual now, but they wouldn't have been at the time that Stonehenge was first constructed. Similar solar calendars have been recorded in Egypt, during a time period there known as the Old Kingdom, and 10-day weeks have appeared in other regions too.

"Such a solar calendar was developed in the eastern Mediterranean in the centuries after 3000 BCE and was adopted in Egypt as the Civil Calendar around 2700 BCE, and was widely used at the start of the Old Kingdom about 2600 BCE," says Darvill.

What's not clear is whether this knowledge could have made it all the way to the south of England at the time. Stonehenge is after all rather unique in its design and construction, and may have been developed entirely by the local population.

Darvill points to a historical figure known as the Amesbury Archer – born in the Alps but later settling in Britain, and buried near Stonehenge – as evidence that travelers might have brought teachings about the intricacies of solar calendar designs with them from the Mediterranean region.

Some of these questions might be answered by future artifact analysis and DNA work, the research suggests. For now, the recognition of Stonehenge as a fully working calendar gives us a better idea of how the people of the time lived and celebrated.

"Finding a solar calendar represented in the architecture of Stonehenge opens up a whole new way of seeing the monument as a place for the living," says Darvill.

"A place where the timing of ceremonies and festivals was connected to the very fabric of the Universe and celestial movements in the heavens."

The research has been published in Antiquity.