The Anno Domini system jumps straight from 1 BC to AD 1 – there is no year 0. So from 1 BC to AD 1 is one year, and from 500 BC to AD 500 is 999 years, not 1,000. Astronomers use a year 0 (1 BC = 0) to make arithmetic simple.
How to use the BC to AD Calculator
- First year
- Era
- Second year
- Era
Results update as you type, and every field accepts the units shown in its menu.
How it works
Enter the first year and choose BC or AD, then the second year and its era.
Tips
- Check whether the end date should be included – event durations usually include it, elapsed time does not.
- Months have 28–31 days, so “one month” is counted by calendar date, not as 30 days.
- Use 24-hour time or set AM/PM carefully when entering clock times.
- Time zones and daylight-saving changes are not applied unless the calculator says so.
Common mistakes to avoid
- Counting a 30-day month when a calendar month is meant (or the reverse).
- Forgetting leap years when counting days across February.
- Mixing decimal hours (7.5 h) with clock time (7:30).
Inputs explained
| Field | What to enter |
|---|---|
| First year | A number |
| Era | Choose one: BC / BCE, AD / CE |
| Second year | A number |
| Era | Choose one: BC / BCE, AD / CE |
What each result means
- Years between = abs([Second year (astronomical)] – [First year (astronomical)])
- First year was this many years ago = year(today()) – [First year (astronomical)]
- Centuries between = abs([Second year (astronomical)] – [First year (astronomical)])÷100
Example values
How the main result changes when First year changes and everything else stays at the example values above:
| First year | Years between |
|---|---|
| 376.5 | 852 |
| 564.75 | 1,040 |
| 753 | 1,228 |
| 941.25 | 1,416 |
| 1,129.5 | 1,605 |
| 1,506 | 1,981 |
Formula
Astronomical year: AD y → y; BC y → 1 − yYears between = later astronomical year − earlier astronomical yearBC → AD span = BC year + AD year − 1Worked example
Inputs — First year: 753; Era: BC / BCE; Second year: 476; Era: AD / CE.
Results — First year (astronomical) = -752; Second year (astronomical) = 476; Years between = 1,228; First year was this many years ago = 2,778; Centuries between = 12.28.
Frequently asked questions
How many years between 753 BC and AD 476?
1,228 years (753 + 476 − 1).
Is there a year 0?
Not in the BC/AD system; astronomers count 1 BC as year 0.
Are BCE and CE different from BC and AD?
No – they are the same years with religiously neutral names.
What formula does the BC to AD Calculator use?
Astronomical year: AD y → y; BC y → 1 − y.