Day: 0
Because of the Portal Network, time keeping in the game universe is complicated. Sometimes portals lead to locations on the same planet, and sometime they lead to planets on the other side of the universe in a different galaxy. All that these planets have in common is that their atmospheres and temperature ranges, flora and fauna, are all mostly within human tolerances (if still often different from Earth, and sometimes very different). Different planets have different lengths of day, different colored skies, different astronomical pictures of the night sky (if they can see the night sky), different seasonal patterns, etc. Even on planets where multiple portals connect different continents, people have had to reckon with the sudden change of night to day, summer to winter, etc.
Because of all the above, time keeping is a mostly local affair. In places where portals are used often, the local cultures have long since found ways to calendarize these inconsistencies. In places where they are rarely used or not nearby, it's not a pressing issue.
The way I will deal with it as GM is through simulation. Whenever I run a simulation in the game universe (by myself or with the main party), the "Day tracker" at the top of this page will advance and I will update the Wiki with anything that's changed. Beyond that, each large area in the Places section will have a section on time keeping, which explains local seasonal patterns (and what the current season / climate is in the relevant area).
Distinct Time Keeping Regions (so far)