What step units are supported?

Days (d, day, days), weeks (w, week, weeks), months (mo, month, months), and years (y, year, years), all case-insensitive, written as a number followed by the unit, like "10 days" or "1 month".

How does it handle month-end overflow, like starting on January 31st and stepping by 1 month?

The day of month is clamped to the target month's actual length. Starting from January 31st and stepping by a month lands on February 28th (or the 29th in a leap year), not an invalid February 31st.

Is there a maximum count?

Yes, 1000 dates per batch. Anything higher is rejected with an error asking for a smaller count.

Do day/week steps and month/year steps advance the same way?

No. Day and week steps add a fixed number of days to the start date's epoch-day count, always landing exactly N days later. Month and year steps advance calendar month by calendar month, reapplying the day-of-month clamp at each step, so the day of month can shift on shorter months.

Generate Custom Dates | Generate a sequence of calendar dates from a start date

Overview

Introduction

Building a list of recurring dates by hand, every other Tuesday, the first of each month, a date every ten days, gets tedious fast and is easy to get wrong around month boundaries.

This tool generates that sequence for you: give it a start date, a step, and a count, and it produces the full list in one pass.

What Is Generate Custom Dates?

A custom calendar date generator that takes a start date, a step expressed as a number and a unit (days, weeks, months, or years), and a count, then outputs that many dates in YYYY-MM-DD form, one per line.

It handles day- and week-based steps with simple day-count arithmetic, and month- and year-based steps with calendar-aware month arithmetic that clamps overflowing day-of-month values.

How Generate Custom Dates Works

The start date is parsed strictly as YYYY-MM-DD and validated as a real calendar date. The step line is parsed into an amount and a unit, with weeks converted to an equivalent number of days and years converted to an equivalent number of months.

For day-based steps, each date is computed by adding a multiple of the step to the start date's epoch-day count. For month-based steps, each date is computed by advancing the calendar month directly and clamping the day of month to the target month's length when needed.

When To Use Generate Custom Dates

Use it to build a list of recurring dates for a schedule, billing cycle, or reminder series, like every 2 weeks or the 15th of every month.

It's also useful for generating test data or fixtures that need a realistic, evenly spaced run of calendar dates.

Often used alongside Generate a Date Sequence, Sort Calendar Dates and Validate a Calendar Date.

Features

Advantages

Correctly clamps day-of-month overflow on month and year steps instead of producing invalid dates like February 31st.
Supports four different step units, days, weeks, months, and years, from a single compact input format.
Generates up to 1000 dates in one batch, well beyond what's practical to type by hand.

Limitations

Caps out at 1000 dates per run; larger sequences need to be split across multiple batches.
Month-based steps that clamp the day of month can drift the day-of-month value over a long sequence, for example a monthly step starting on the 31st will land on shorter months' last day, not always the 31st.

Examples

Generating 5 dates, 10 days apart

Input

2024-01-01
10 days
5

Output

Each date is exactly 10 days after the previous one, computed by adding whole days to the epoch-day count.

Generating monthly dates across a leap-year boundary

Input

2024-01-31
1 month
3

Output

2024-01-31
2024-02-29
2024-03-29

January 31st plus 1 month clamps to February 29th, 2024 being a leap year; adding another month from a clamped date carries the clamp forward to March 29th.

Best Practices & Notes

Best Practices

For monthly or yearly recurring schedules where the exact day of month matters most months, start from a day near the beginning of the month to avoid repeated clamping on shorter months.
Use a day- or week-based step instead of a month-based one when you need a perfectly even day-count gap between every date, since month steps can vary in actual day length.

Developer Notes

Day and week steps use `epochDayFromDate` and `dateFromEpochDay` helpers built on `Date.UTC(...) / 86400000`, so every date lands exactly N whole days apart regardless of calendar irregularities. Month and year steps use an `addMonths` helper that computes `totalMonths = (start.month - 1) + months`, derives year and month from it, then clamps `day` to `Math.min(start.day, daysInMonth(year, month))`.

Generate Custom Dates Use Cases

Generating a recurring billing or invoice date schedule
Building a list of reminder or check-in dates spaced a fixed interval apart
Producing realistic date sequences for test fixtures or sample data

Common Mistakes

Assuming a monthly step always preserves the exact day of month; a step that starts on the 31st will clamp down on any month shorter than 31 days.
Requesting more than 1000 dates in a single run, which is rejected; split large sequences into multiple batches instead.

Tips

If a monthly sequence needs to stay locked to the last day of each month regardless of length, start the sequence from the last day of the first month rather than a fixed day like the 31st.
Combine week-based steps with a small count to quickly sanity-check a biweekly or triweekly schedule before committing to it elsewhere.

Generate Custom Dates