The calendar in the RuneScape multiverse has ten months, as revealed in Postbag from the Hedge issue 21. The total number of days in a year on Gielinor is 365.25 (see "leap year"), as with the real world.
The current RuneScape calendar date is 20 Septober. The year did not appear to change over time in sync with real world years within the RuneScape storyline until the Sixth Age began. Until the events of The World Wakes, it had been year 169 of the Fifth Age since the release of the game in 2001. The death of Guthix in The World Wakes, released on 4 March 2013, marked the first day of year 1 of the Sixth Age. The Sixth Age calendar was first revealed to have advanced in Sliske's Ramblings, which gave rough dates for certain Sixth Age events, including the Battle of Lumbridge, the Bird and the Beast, and Sliske and the Chocolate Factory.
It is understood that the flow of time in RuneScape need not always directly correlate with the flow of time in real life. It has been confirmed in a developer livestream that time moves in-game only if something is done in-game which advances the RuneScape storyline, such as quests or holiday events. NPC dialogues and lores, among others, may provide dates that give an indication of when events have happened in-game on the calendar within the RuneScape multiverse.
Should the player complete quests or other in-game activities in a non-chronological order, it is understood that previous events are understood as having taken place in the past; now that Gielinor is currently in the Sixth Age, for instance, quests that took place in the Fifth Age are still playable, but many of these quests are marked specifically as having taken place in the past, at a time when the Edicts of Guthix were still in place and the gods had not yet returned to Gielinor.
Postbag from the Hedge
|“|| Presently, we are in the 169th year of the 5th Age. The length of a day, of course, is equal to one cycle of the sun and the moon.
There are ten months in the RuneScape calendar. [...] An extra day is added to Moevyng every four years to account for an anomaly in the way the calendar has been calculated. Moevyng Day, as it is called, tends to fall in the middle of the month.
The most intriguingly-named month is the Ire of Phyrrys. My colleagues over at the Observatory have uncovered the charred remains of a diary that belonged to Phyrrys, apprentice to the astronomer Scorpius.
Early entries in Phyrrys's diary tell of how he was attempting to figure out the RuneScape calendar, based upon the astronomical studies his mentor was conducting. However, a few pages have survived of Scorpius's own diary, heavily hinting at Phyrrys's ineptitude, and how at the culmination of the apprentice's initial calendar research, he'd failed to account for a whole forty days.
This is backed up in Phyrrys's own diary entries, whereby from this date on, his anger (ire) seemed to grow and grow at his inability to work these forty days into the calendar. It is assumed that a later inhabitant of the Observatory found these notes and, quite easily, corrected the mistake by placing them between Septober and Novtumber, naming the month in honour (or more likely in jest) of the 'challenged' Phyrrys.
— Reldo 
Based on this information, the total number of days in the year is 365, which suggests that the Gielinorian calendar and the real-life Gregorian calendar indeed overlap.
Months of the year
The months according to the RuneScape calendar as used on Gielinor are:
|Month||Number of days||Real-world equivalent||Details|
|1||Rintra||39||1 January – 8 February||Tied for 2nd longest month with Novtumber and Moevyng on years with Moevyng Day.|
|9 February – 18 March||There is an additional day in the middle of Moevyng every four years, called Moevyng Day, similar to a leap day.|
|3||Bennath||32||19 March – 19 April||Bennath means "blessing" in Cornish. 2nd shortest month.|
|4||Raktuber||34||20 April – 23 May||The Red Raktuber (which is a submarine) is named after this month.|
|5||Pentember||38||24 May – 30 June||Named for its position as the fifth month. Pente (πέντε) is Greek for five.|
|6||Fentuary||31||1 July – 31 July||Only month to solely encompass an entire Gregorian month. Shortest month.|
|7||Septober||38||1 August – 7 September||Named for its position as the seventh month. Septem is Latin for seven.|
|8||Ire of Phyrrys||40||8 September – 17 October||The longest month on the calendar, which is named after the apprentice of Scorpius, Phyrrys. Phyrrys began work on the calendar of RuneScape.|
|9||Novtumber||39||18 October – 25 November||Named for its position as the ninth month. Novem is Latin for nine.|
|10||Wintumber||36||26 November – 31 December||Named after winter; Wintumber trees are named after this month.|
Saradominist days of the week
The "Old Saradominist calendar", as mentioned by a H.A.M. guard, is a different system of names for the days of the week as seen in The Chosen Commander. It seems to have fallen out of common use at some point, but some people, such as Nora T. Hagg and a H.A.M splinter group still seem to use it, however. The names for the days and their modern equivalents are:
- Ivanday - Monday
- Caistleday - Tuesday
- Duneday - Wednesday
- Gullday - Thursday
- Erysail - Friday
- Twiblick - Saturday
- Essianday - Sunday
Ivanday, Erysail, Twiblick, Essianday (four of the seven Saradominist days of the week) were used in a RuneScape poll question on 29 June 2010. The question was: What do the words "Ivanday", "Erysail", "Twiblick" and "Essianday" refer to?