# day (2-day.us)

A day is the time between sunrise and sunrise, about twenty-four hours. People use it to plan schedules, count deadlines, and talk about routines.

## What a model may hear
- Julian day number (astronomy and date libraries): Convert a calendar date to a serial day count starting from 4713 BCE, or reverse it
- day of week (scheduling and calendar systems): Extract Monday through Sunday from a date, or filter events by weekday
- daylight period (solar calculations and weather data): Compute sunrise, sunset, and hours of light for a location and date
- trading day (finance and markets): Skip weekends and holidays when counting business dates or settlement periods
- day as integer (SQL and databases): Extract the day-of-month component from a datetime field, returning 1 through 31

## Where people and models part ways
- Says: "What day is it?"
  Means: What is today's date and day of the week
  May be taken as: Return the Julian day number or a day-of-year integer
  Say instead: "What is today's date and what day of the week is it"
- Says: "In two days"
  Means: The day after tomorrow, calendar time
  May be taken as: Count two business days, skipping the weekend, or add forty-eight hours exactly
  Say instead: "Two calendar days from now, including weekends"
- Says: "Day and night"
  Means: The full twenty-four hour cycle
  May be taken as: Separate into daylight hours and nighttime hours using solar calculations
  Say instead: "The full twenty-four hour period, not split by sunlight"
- Says: "Same day delivery"
  Means: Arrives today, this calendar day
  May be taken as: Interpret as same business day, which may exclude weekends or end at market close
  Say instead: "Arrives today, this calendar day, including weekends"

## Tips
- Say 'calendar day' when you mean any day of the week
- Say 'business day' or 'weekday' when you want to skip weekends
- Specify a time zone when asking about sunrise, sunset, or 'today'
- Say 'twenty-four hours' when you mean an exact duration, not a calendar date
- Say 'day of month' when you want a number like the 15th, to avoid Julian day confusion

## Often confused with
- date: includes month and year, not just day
- daytime: only the light hours, not the full cycle
- daily: frequency, not a specific twenty-four hour block
- daylight: illumination period, not a time unit
- dawn: a moment, not a full day
- epoch: a fixed starting point, not a repeating unit