Tuesday: Ruby-Compatible Strftime for Go

Tuesday formats Go time.Time values with Ruby-compatible strftime format strings. It supports padding and case flags, field widths, fractional seconds, epoch time, and colon-delimited timezone offsets.

Tuesday was developed for use by Liquid and Gojekyll.

Install

go get github.com/osteele/tuesday@latest

Usage

package main

import (
	"fmt"
	"time"

	"github.com/osteele/tuesday"
)

func main() {
	value := time.Date(2026, 8, 11, 15, 4, 5, 123456789, time.UTC)
	formatted, err := tuesday.Strftime("%Y-%m-%d %H:%M:%S.%3N %:z", value)
	if err != nil {
		panic(err)
	}
	fmt.Println(formatted)
}

Output:

2026-08-11 15:04:05.123 +00:00

Compile formats that are used repeatedly. A compiled formatter is immutable and safe for concurrent use:

formatter, err := tuesday.Compile("%a, %b %d, %Y")
if err != nil {
	panic(err)
}
formatted := formatter.Format(value)

Compatibility

Tuesday targets the formatting behavior of Ruby 3.4 Time#strftime. Its differential test matrix also uses DateTime#strftime for %Q, which Ruby Time does not implement. Tuesday additionally supports %+ as an extension; Ruby Time does not implement %+, while Ruby DateTime prints the UTC offset rather than the location name.

• Month and weekday names use Go's English names, corresponding to Ruby's C locale. Locale-specific names are not supported; E and O modifiers use the corresponding unmodified conversion.
• %Z uses the name attached to the Go time.Location. A numeric offset alone does not imply a timezone abbreviation.
• Unsupported directives are copied to the result unchanged.
• Supported field widths greater than 1 MiB return an error to prevent excessive allocation.
• %Q, %N, %L, %:z, %::z, and %:::z follow the corresponding Ruby DateTime, fractional-second, and timezone conventions.

The Ruby differential test is skipped when Ruby is unavailable. The checked-in Go tests remain sufficient to run the package test suite without Ruby.

Development

go test ./...
go test -fuzz=FuzzStrftime
go test -bench=. -benchmem

References

• Ruby 3.4 Time
• Ruby 3.4 DateTime

License

MIT License