Just to rephrase Niantic's official statement: this badge is like a rocket in time. Its petrol tank can only last for 24h, every hack you do fills it up completely. If you don't fill up for more than 24h the rocket crashes, and you have to restart again. The badge counter displays the longest distance in days your rocket has ever travelled.

Max 36 hours, any portal, only successful hacks count

NOTE: These "rules" are not officially supported and may change at any time!

The only rule that is confirmed by Niantic is "max 24 hours per hack", although they have admitted there is a little bit of grace, but they never confirmed what that grace is.

I just did some testing on this, and the rules appears to be as follows:

• You gain Sojourner medals for achieving a Hack Steak of 15, 30, 60, 180 and 360 days
• Hacking on any portal counts
• Only successful hacks count; failures due to full inventory does not count
• Max 36 hours between hacks to maintain the Hack Streak

So this is the series of hacks I did, and the corresponding Hack Streak counter

• Sunday, 20:12, 0 days
• Monday
• Tuesday, 08:10, 1 day
• Wednesday, 20:08, 2 days
• Thursday
• Friday, 08:06, 4 days
• Saturday, 20:04, 5 days
• Sunday
• Monday, 08:02, 7 days
• Tuesday, 19:00, mistaken failed hack (inventory was full), 7 days (should have been 8)
• Tuesday, 20:04, successful hack, 0 days

It might be worth noting that the Hack Streak counter said 8 days in the seconds just before the Tuesday 20:04 hack, and was then 0 directly after.

Notes on the Hack Streak counter

• The Hack Streak counter is unreliable. It can say the Hack Streak is still on when it has in fact been reset
• The time of day of the very first hack only tells when the hack streak counter will tick over after on a successful hack
• The tick-over time is UTC, so if you started at 13:00 standard time, it will tick over on 14:00 in daylight saving time
• The counter may tick over despite you not having hacked for that period, so you cannot use that as an indicator to recall if you have hacked successfully recently or not

The text of badge is not very clear, but to be sure of keeping the number growing you should ensure the time between two hacks is never longer than 24h.

This way, whatever the exact time in the day is the limit between 24h periods, you are ok.

If you are on holiday and need 1h walk to go to the nearest portal, make sure you go there each day earlier than previous one. You may have to do the trip twice in a day once in a while.

If it was limited to 24 hours, you would have to hack portals before the time limit, and you would incur drift back towards the original time. 24 hours leeway would account for this, but, if applied every day would allow drift in the other direction. So, it seems that a 24 hour timer would start the first day, with a check to make sure you didn't accumulate the spare 24 hours while you waited.

Basically, you should get 48 hours to make the hack, but not on more than one consecutive day.

Otherwise, you would hack at 10pm the first day, 9pm the second day to make sure you weren't late, then drift back 24 hours in 24 days till you had to hack twice to catch up, which is more than the badge requires.

Hopefully someone not half asleep can make sense of that for me :-)

36 hours since your last hack. A 36 hour countdown is set each time you hack. If it runs out before hacking again, you lose your streak.

By experimentation, I have determined that the official time requirement between hacks is exactly 36 hours. My dataset consists of a hack at 9:40 AM Thurs (Thanksgiving 2016). Then a hack at 9:20 PM Friday. Then a hack at 9:10 AM Sunday. Each hack was just shy of 36 hours apart and I covered 72 hours in total. Also notice that Saturday was skipped altogether. I kept the streak up til the following weekend by hacking every 24 hours, then tried a 47 hour gap, which failed.

I have discovered that although I may miss the 24 hour mark... sometimes when logging in it would say the "you've been gone for 30 hours... I miss you... etc.." and after hacking it continues to count... I expect, due to time zones they either provide padding to encourage people, or there is a bug where they just ensure there is only 1 day apart and don't take hours into account. I'm certainly not complaining! Because what about the person who Hacks in America, then flies around the world to Australia, and hacks there, I guess it's just server time, but still can get confusing. And I would think it's better for them to encourage people to continue playing.

(This answer is a duplicate of this one, after a discussion on meta)

Given my observations, it's your first bullet point: when you hack a portal on a particular day.

I know, this contradicts the official answer from NIA as cited in the top-voted answer, but this has been tested IRL.

Observations: I have, on two occasions, made a hack on Friday afternoon at 3-4pm, and the next hack on Saturday evening at 8-9pm. In both cases, I've kept my current hacking streak alive.

My theory: The interval you need to hack in depends on when you made your first hack in the current streak (and not your latest hack, as NIA states). For me, my first hack in this streak was at 6 am. This means that I had to make a hack between Friday 6 am - Saturday 5.59 am, and the next one between Saturday 6am - Sunday 5.59 am. As long as I make at least one hack during each period, I will keep the hacking streak alive. The fact that the particular Friday hacks were at 3-4 pm is irrelevant, they could have been made at 7 am or 2 pm as well.

This theory also fits with the experience people here and IRL have had, that the time their current hacking streak increases (or Sojourner badge is awarded) is different for everyone. For me, this happens early in the morning, likely at around 6 am.

I believe it works more or less like this : you start a streak with the first hack lats say on 1 jan 13:00 , your first block is till 2 jan 13:00, so your next 24 hour block is from 2 jan 13:00 til 3 jan 13:00 in this block you have to hack a portal to keep the streak active , and so on