Common date-conversion mistakes and how to avoid them

Converting between the Shamsi (Jalali) and Miladi (Gregorian) calendars is straightforward once you know where people usually slip up. Most mistakes come from treating the two calendars as if they lined up neatly — they do not. Here are the errors we see most often, why they happen, and how to get the right answer every time.

Assuming a fixed offset between the calendars

A common shortcut is to "just subtract 621" from a Shamsi year to get the Gregorian one. That works for part of the year, but it breaks the moment you cross Nowruz (1 Farvardin, around 20–21 March). Before Nowruz a Shamsi year overlaps with one Gregorian year; after Nowruz it overlaps with the next. There is no single number you can add or subtract that is correct for every date.

The same applies to days. The two calendars have different month lengths and different leap-year rules, so a fixed day offset drifts over time. Always convert the full date — year, month and day together — rather than adjusting one part by hand. Our converter does this for you.

Confusing 1 Farvardin with 1 January

The Persian year does not start on 1 January. It starts on Nowruz, 1 Farvardin, which falls around 20–21 March. So the first day of a Shamsi year sits roughly three months into the Gregorian year. Treating 1 Farvardin as if it were 1 January (or vice versa) throws every later calculation off by about a quarter of a year.

The off-by-one year at the Nowruz boundary

This is the single most frequent error. A date in late winter can belong to one Shamsi year but the next Gregorian year, and the line between them is Nowruz. For example, 30 Esfand 1399 = 20 March 2021 — the Shamsi year is 1399, but the Gregorian year is already 2021. A day later it becomes 1 Farvardin 1400 = 21 March 2021. If you guess the year instead of converting the exact day, you will often be off by one.

Some reliable anchor points: 1 Farvardin 1403 = 20 March 2024 and 1 Farvardin 1404 = 21 March 2025. Even Nowruz itself shifts by a day between years, which is exactly why a fixed rule fails.

Mishandling leap-year Esfand

Esfand, the last Persian month, normally has 29 days but gains a 30th day in a leap year. People often assume Esfand always has 30 days (or always 29) and then enter a date that does not exist — such as 30 Esfand in an ordinary year. Persian leap years follow an arithmetic cycle, not the simple Gregorian "divisible by 4" rule, so you cannot reliably guess which years are leap years. If you are unsure, see our guide to leap years in both calendars, or just let the converter tell you whether the date is valid.

Getting the day, month and year order wrong

Many dates are written as three numbers, and the order is not always obvious. The converter reads pasted dates intelligently, but it can only do so when the order is unambiguous:

• If the first number is greater than 31, it is read as year-month-day (for example, 1403/01/20 style input).
• If the last number is greater than 31, it is read as day-month-year.
• If all three numbers are 31 or less, the order is genuinely ambiguous and the converter will not guess — it returns nothing rather than risk a wrong answer.

To avoid this, write the year in full and put it first or last, so at least one number is clearly above 31. The calendar is then detected automatically: a year up to 1700 is treated as Shamsi, and a larger year as Gregorian. You can also type Persian digits (۱۴۰۳) — they are converted automatically.

Letting time zones shift the date

Online tools that work with timestamps can quietly move a date across midnight depending on your time zone, turning, say, 20 March into 19 March. This converter uses a date-only model with no time component, so time zones never change the result. A calendar date is a calendar date, wherever you are.

A quick checklist

1. Convert the whole date at once — never adjust a single part by a fixed offset.
2. Remember the year starts at Nowruz, not 1 January.
3. Watch for the off-by-one year around late Esfand and early Farvardin.
4. Check whether the year is a leap year before entering 30 Esfand.
5. Write years in full so the day/month/year order is unambiguous.

When in doubt, paste or type the date into the converter — it cross-checks both calendars and only accepts a date that genuinely exists. You may also find the step-by-step guides for Shamsi to Gregorian and Gregorian to Shamsi helpful.