Days Since and Until Calculator

Enter Calculation

Year

Month January February March April May June July August September October November December

Day

From Year (reference)

From Month January February March April May June July August September October November December

From Day

Formula

Show calculation steps (1)

1. Weeks Between

   

   Number of whole-and-fractional weeks between the two dates, using the absolute day difference.

Results

/* keep markers below */

Days Since Target Date

183

days in the past

Signed difference (negative = past, positive = future)	-183 days
In weeks	26.14 weeks
Direction	Past

What this calculator does

The Days Since and Until Calculator tells you exactly how many days separate a target date from a reference date. If the target date is in the future you get a positive "days until" count; if it is in the past you get a negative "days since" count. It works for any pair of dates — birthdays, deadlines, anniversaries, contract dates, project milestones, or historical events. The tool is calendar-based and universal, so it applies anywhere in the world with no country-specific rules.

Days since are counted to the left (past) and days until to the right (future) of the reference date.

How to use it

Enter the target date (the date you are counting toward or from) using the year, month, and day fields. Then enter the reference date — by default this is today, but you can set it to any date to compare two arbitrary dates. The calculator returns the absolute number of days, a signed difference (negative for the past, positive for the future), and the equivalent number of weeks.

The formula explained

Each date is converted to a midnight timestamp in milliseconds. The difference between the two timestamps is divided by 86,400,000 (the number of milliseconds in one 24-hour day) and floored to a whole number:

$$\Delta = \left\lfloor \frac{\text{Target Date} - \text{Reference Date}}{86400000\ \text{ms}} \right\rfloor$$

Because both dates are pinned to midnight, the result is the clean count of whole calendar days between them.

The difference in milliseconds between two dates divided by 86,400,000 gives whole days.

Worked example

Reference date: 1 January 2025. Target date: 31 January 2025. The difference is 30 days in the future, equal to about 4.29 weeks. If you swapped the dates — reference 31 January, target 1 January — the signed result would be −30 days (in the past), while the absolute count stays at 30 days.

$$\text{Weeks} = \frac{\left| \Delta \right|}{7} = \frac{30}{7} \approx 4.29$$

FAQ

Does it count today? The difference is exclusive of the start date: from today to tomorrow is 1 day. Same-date inputs return 0.

Does it handle leap years? Yes. The calendar engine accounts for February 29 automatically.

Can the reference date be in the future? Absolutely — you can compare any two dates in either direction; the sign tells you which way.