# day (7-day.us)

A day is the time the sun rises and sets once, about twenty-four hours. People count days to plan ahead, to wait, or to mark how long something lasts.

## What a model may hear
- Julian day number (astronomy and date libraries): treat it as a continuous count of days since noon UTC January 1, 4713 BC, used to calculate exact intervals
- day-of-week enum (programming (Python datetime, Java DayOfWeek)): return an integer 0-6 or 1-7, where Monday may be 0 or 1 depending on locale
- business day or trading day (finance and scheduling software): exclude weekends and holidays, so 'in 7 days' and 'in 7 business days' differ
- daylight saving transition day (timezone databases (IANA tzdb)): flag a date where the day is 23 or 25 hours long, not 24
- day as a unit in pandas or R (data science): round timestamps to calendar dates, or group data by date ignoring time-of-day

## Where people and models part ways
- Says: "What day is it?"
  Means: What is today's date or day of the week?
  May be taken as: the model may answer with a Julian day number or Unix epoch day
  Say instead: "What is today's date and day of the week?"
- Says: "Remind me in three days"
  Means: Remind me after three calendar days pass
  May be taken as: the model may schedule using business days or exact 72-hour intervals
  Say instead: "Remind me on Thursday if today is Monday, regardless of work schedule"
- Says: "Group sales by day"
  Means: Show me total sales for each calendar date
  May be taken as: the model may group by 24-hour rolling windows or by business days only
  Say instead: "Sum sales for each calendar date, from midnight to midnight in my timezone"
- Says: "How many days between these dates?"
  Means: Count the calendar dates spanned
  May be taken as: the model may compute exact 24-hour periods or exclude the start date
  Say instead: "Count each calendar date from the start date through the end date inclusive"

## Tips
- Say 'calendar day' when you mean any day of the week, to avoid business-day assumptions
- Give a specific date like 'March 15' instead of 'in ten days' if the exact date matters
- Mention your timezone when asking about dates near midnight or daylight saving changes
- Say 'midnight to midnight' if you need strict 24-hour periods rather than calendar dates
- Specify 'including weekends' or 'business days only' for scheduling tasks

## Often confused with
- date: a specific point on a calendar, while day can mean a duration or a day-of-week
- daily: habitual or recurring, while day is a single unit
- weekday: only Monday through Friday, excluding Saturday and Sunday
- 24 hours: an exact duration, while a calendar day may be 23 or 25 hours
- epoch: a fixed reference point in computing time, not a human calendar unit
- business day: excludes weekends and holidays, unlike a plain calendar day