Time Flies Campaigns
The Time Flies rule states that during a campaign, the timeline always advances between game sessions by a period of not less than twice the real-world time that has passed. If it has been a week since you last played, then two weeks passed in the game world; if it has been a month, then two months have passed. Just add the time to the game’s calendar, noting that it includes rest and recovery time or downtime or heroes-on-vacation time If the group is in a dungeon or on a highly time-sensitive mission, apply the additional time once they return to a suitable location.
For campaigns that seek to emulate the storytelling rhythm of a long-form saga, consider increasing this modifier to at least two months of game time between sessions. As a result, characters can age at a noticeable rate (six game sessions equal one year of game time), and generational effects become more pronounced. In addition, this prevents the chronological oddity of heroes going from novice to archmage in only a year or two.
Source: Midgard World Book (Open Design), pg. 13
GM Note
So how does this actually work when when a game has to stop in the middle of a combat, etc. you are probably wondering...? Well what happens behind the scene is that if a session stops due to time constraints, fatigue, etc.; the GM will record the amount of time passing and then add that in as downtime available to players at convenient points (while doing long overland travel a day here or there might be added; when resting in an urban area or sanctuary as much as a week or more will be added).