You’re probably here because you looked at a calendar, felt a weird sense of vertigo, and realized the end of the year is barreling toward us. It happens. We hit that mid-January slump and suddenly start counting backward to the last time things felt "normal" or when a specific season shifted. Calculating the days since Sept 29 isn't just a math problem for people trying to track a project deadline; it’s a weirdly specific cultural and chronological marker that anchors the transition from the warmth of late September into the teeth of winter.
Time is slippery.
If we're looking at today—January 12, 2026—we are exactly 105 days removed from September 29. That’s roughly three and a half months. It feels like a lifetime ago, doesn't it? Back then, the Northern Hemisphere was just kissing summer goodbye. Now, we’re deep in the "New Year, New Me" cycle, likely already failing at half our resolutions. But that 105-day gap represents more than just a digit on a calculator. It’s the entire arc of the holiday season, the shift in fiscal quarters, and for a lot of people, the duration of a habit-forming cycle.
Why the Days Since Sept 29 Matter for Your Body
Biologically, 105 days is a massive window. You've likely heard the old myth that it takes 21 days to form a habit. Well, researchers like Philippa Lally at University College London have largely debunked that, showing it actually takes closer to 66 days on average—and sometimes up to 254 days—for a behavior to become automatic.
If you started a fitness kick or a lifestyle change on September 29, you are currently in the "make or break" zone. At 105 days, you’ve passed the two-month hump. You're deep into the territory where the initial excitement has died, and pure discipline has to take over. Most people quit by day 30. If you’re still going, your neural pathways are literally reconfiguring themselves.
The weather plays a part too. Since late September, the amount of daily sunlight in the Northern Hemisphere has plummeted. We’ve passed the winter solstice. We are now in that strange period where the days are technically getting longer, but it feels colder than ever. Tracking the days since Sept 29 helps some people manage Seasonal Affective Disorder (SAD) by visualizing where they are in the dark half of the year.
The Fiscal and Business Reality
Business owners look at this date range through a totally different lens. September 29 often marks the very end of Q3 or the dawn of Q4. It’s the "crunch time" start date.
Think about it.
When a company looks at their performance 105 days after September 29, they are looking at the wreckage or the triumph of the holiday retail season. Black Friday, Cyber Monday, and the December rush all live within this specific window. For a small business, this duration determines whether they’re in the black or the red for the entire year. It’s a period of intense volatility. Supply chains are stressed. Shipping rates fluctuate. If you ordered something on September 29 and it still hasn't arrived? Yeah, you’ve got a logistics nightmare on your hands.
Tracking the Astronomical Shift
Astronomically, September 29 is usually just a few days past the Autumnal Equinox. This is when day and night are roughly equal in length. Since then, the Earth has tilted significantly. We’ve swung through the perihelion—the point in our orbit where we are actually closest to the Sun (which happened in early January), despite the shivering temperatures in New York or London.
It’s a paradox of physics.
We are closer to our star now than we were 105 days ago, yet the tilt of the axis means the energy is spread thin across the northern latitudes. People often use "days since" calculators to track astronomical events or even satellite de-orbits. If a piece of space debris was flagged on September 29, its decay rate over 105 days provides crucial data for orbital mechanics experts at places like NASA or the ESA.
The Human Element: Grief and Milestones
On a more personal, kinda heavy note, people track days since Sept 29 for reasons that don't involve spreadsheets. It could be the day a loved one passed, the day a relationship ended, or the day someone took their last drink.
In recovery circles, 100 days is a monumental milestone.
Crossing that century mark—which happened just five days ago if you started on September 29—is often celebrated with chips, meetings, or just a quiet sense of "I actually did it." It’s the point where "not doing the thing" becomes the new normal. If you’re at day 105, you’ve survived the gauntlet of Thanksgiving, Christmas, and New Year’s Eve sober. That is a statistical anomaly. Most people relapse during the holidays. Surviving those 105 days is basically a superpower.
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Surprising Facts About Late September Starts
- The 100-Day Rule: Many political pundits judge the success of a new initiative or a local government shift by the first 100 days. If a policy was enacted on September 29, its "grace period" officially expired last week.
- Academic Burnout: For students, September 29 is usually the end of the "honeymoon phase" of the fall semester. 105 days later, they are either in the middle of winter break or staring down the barrel of a new term, processing the grades from the work they started back then.
- Agriculture: Farmers in many regions use this window to track soil rest. From the final harvest around late September to the deep freeze of mid-January, the land is undergoing chemical changes that prepare it for spring.
It’s easy to think of time as a straight line, but it’s more like a series of nested loops. Every day we move further from September 29, we are moving closer to the next one. But the context changes. Right now, we are in the "waiting room" of the year. February is around the corner, which is notoriously the most "nothing" month for many.
What You Should Do With This Information
Honestly, don't just let the number sit there. If you’re tracking days since Sept 29 for a project, look at your velocity. If you’ve only finished 20% of your goal in 105 days, you aren't going to finish by spring without a radical shift.
Here is how to actually use this milestone:
Check your "Long-Term" habits. If you started something on September 29, evaluate it today. Is it serving you? If it’s a bad habit you haven't broken yet, realize you’ve let 105 days slip by. That’s a quarter of a year. Use the "Day 105" realization as a jolt to the system.
Audit your finances. Look at your bank statements from late September versus today. The "holiday bleed" is real. Most people spend 30% more in this 105-day window than any other time of year. It’s time to tighten the belt.
Lastly, acknowledge the tilt. The days are getting longer. Even if it’s by seconds, the trajectory has shifted. We are 105 days out from the start of autumn, which means we are that much closer to the first signs of life in the dirt.
Stop counting the days and start making the days count. It’s a cliché because it’s true. Whether you're tracking a pregnancy, a dry spell, or a technical deadline, the 105 days behind you are a data set. Use that data to predict your next 100.
Take a look at your calendar right now. Mark 100 days from today. That’s your next checkpoint. If you start a new goal today, January 12, you'll be hitting your century mark in late April. Imagine where you could be when the flowers actually start blooming.