What is date interpolation?

Date interpolation is the process of finding a specific date that lies at a proportional position within a given date range. For example, the date at 25% of a year represents one quarter of the way through, the date at 50% represents the midpoint, and the date at 75% represents three quarters through. This concept is used in project planning, financial reporting, contract management, and anywhere timing milestones need to be expressed as fractions of an overall period.

Unlike simply adding a fixed number of days, percentage-based interpolation automatically adapts to ranges of any length — a week, a quarter, a decade — making it a flexible and universally applicable calculation.

Tool description

This tool calculates the exact date that falls at any percentage point between two given dates. Enter a start date, an end date, and slide the percentage slider to any value between 0% and 100%. The tool instantly returns the date at that position along with three supporting statistics: the total number of days in the range, how many days have passed from the start to the result date, and how many days remain from the result date to the end.

The percentage can be set to any decimal value (e.g., 33.3% for a one-third mark or 66.6% for two-thirds), not just round numbers. The slider and numeric input stay in sync, so you can use either for fine-grained or broad adjustments.

Features

  • Accepts any two dates in any order — the tool automatically identifies which is the start and which is the end
  • Percentage can be any decimal value from 0 to 100, enabling precise fractions like 33.33% or 12.5%
  • Displays total days, days elapsed from start, and days remaining to end alongside the result date
  • Result date is copyable with a single click
  • Instant recalculation on every input change — no submit button needed

Use cases

  • Project management: Find the date that marks the 25%, 50%, or 75% completion milestone of a project timeline
  • Financial quarters: Quickly determine the start or end of fiscal quarters within a custom accounting period
  • Contract milestones: Identify interim review or payment dates at specific proportional points within a contract's duration

How it works

Given a start date $S$ and end date $E$, the tool converts both to Unix timestamps (milliseconds since epoch). The result timestamp is computed as:

$$R = S + \left(\frac{p}{100}\right) \times (E - S)$$

where $p$ is the percentage. The timestamps for the start, result, and end are then used to derive the day counts displayed in the output. Dates are always processed in chronological order regardless of which field was filled first.

FAQ

What happens if I enter the same date for start and end? The total days will be 0 and the result date will equal both dates regardless of the percentage.

Can I use dates further than a year apart? Yes — the tool works across any date range, from a single day to multiple decades.

Is the percentage inclusive of 0% and 100%? Yes. At 0% the result equals the start date; at 100% it equals the end date.