Date & Time Calculator

Last updated: August 2026

The quick answer: Jan 15 + 45 days = Mar 1, and 14:30 + 3 h 45 min = 18:15.

Add or Subtract Time Time Duration (Between Times) Add Time to a Date

Use the Date & Time Calculator below — enter your dates and times, then click Calculate to get the result.

Start Date

Start Time (optional)

24h

Operation

Days

Hours

Minutes

How to Add Days to a Date Manually

Adding days to a date is tricky because months have different lengths — 30, 31, or 28 days (29 in a leap year). To do it by hand, count forward through each month: adding 45 days to March 1 lands on April 15 because March has 31 days (March 1 + 31 = April 1, then 14 more days = April 15). A date calculator does this automatically, which is why it is the fastest and most accurate way to get results like "what is 90 days from today".

Example: Your project ships in 45 days. Today is March 1. March has 31 days, so day 31 lands on April 1, and the remaining 14 days land on April 15. This calculator gives the same result instantly, including the weekday.

Days in Each Month

Month	Days
January	31
February	28 (29 in a leap year)
March	31
April	30
May	31
June	30
July	31
August	31
September	30
October	31
November	30
December	31

Leap Years Explained

A leap year has 366 days instead of 365, adding February 29. Leap years happen every 4 years, except centuries that are not divisible by 400 — so 2000 was a leap year but 2100 will not be. The most recent leap year was 2024; the next is 2028. This calculator handles leap years automatically when you add or subtract large numbers of days.

Frequently Asked Questions

How do I add days to a date?

The easiest way is a date calculator: enter the start date and the number of days, and it returns the exact result date, automatically handling month lengths and leap years. For example, adding 45 days to March 1 gives April 15.

What day is 90 days from today?

Pick today as the start date, add 90 days, and the calculator shows the exact date and weekday. Because months run 28-31 days, the result shifts by calendar position — never assume 90 days equals exactly 3 months.

Why do months have different numbers of days?

April, June, September, and November have 30 days; February has 28 (29 in a leap year); all other months have 31. The pattern comes from the ancient Roman calendar and has stayed roughly the same for over 2,000 years.

How many days are in a year?

A common year has 365 days; a leap year has 366. Leap years occur every 4 years except centuries not divisible by 400 — 2024 was a leap year, 2028 is the next one.

What time is it 45 minutes after 10:15?

11:00. Adding 45 minutes to the 15-minute mark reaches 60 minutes, which carries over to 1 hour — 10:15 + 45 min = 11:00. The same carry-over logic applies to hours reaching 24, which rolls the date forward.

Data sources: Gregorian calendar rules per the International Standard ISO 8601, U.S. Naval Observatory astronomical calendar data, and NIST time standards.

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Date Quick Facts

365 days in a year, 366 in a leap year
Next leap year: 2028
Feb = 28 days, 29 in a leap year
30-day months: Apr, Jun, Sep, Nov
90 days from today = ~13 weeks
7 days in a week = 168 hours