# datefixR: Comprehensive Date Standardization Guide

    library(datefixR)

# Introduction

datefixR is a comprehensive R package designed to automatically standardize messy date data into consistent, machine-readable formats. Whether you’re dealing with mixed date formats from web forms, international datasets, or legacy systems, `datefixR` intelligently parses diverse date representations and converts them to R’s standard `Date` class.

## Key Features

- Smart parsing: Handles mixed date formats, separators, and representations in a single dataset
- Multilingual support: Recognizes dates in English, French, German, Spanish, Portuguese, Russian, Czech, Slovak, and Indonesian
- Missing data imputation: User-controlled behavior for incomplete dates (missing days/months)
- Detailed error reporting: Identifies exactly which dates couldn’t be parsed and why
- Excel compatibility: Supports both R and Excel numeric date representations
- Roman numeral support: Experimental support for Roman numeral months

## Core Functions

The package consists of three main user-accessible functions:

- `fix_date_char()`: Converts character vectors of dates
- `fix_date_df()`: Processes date columns in data frames or tibbles
- `fix_date_app()`: Interactive Shiny application for date cleaning (see dedicated vignette)

## When to Use datefixR

This package is particularly valuable when:

- Processing dates from free-text web form entries
- Handling international datasets with mixed date formats
- Cleaning legacy data with inconsistent date representations
- Working with Excel imports containing mixed text and numeric dates
- Validating and standardizing date data quality

# Core Workflow

### Date standardization

Firstly, we will demonstrate date standardization without imputation. We consider a data frame with two columns of dates in differing formats with no missing data.

    bad.dates <- data.frame(
      id = seq(5),
      some.dates = c(
        "02/05/92",
        "01-04-2020",
        "1996/05/01",
        "2020-05-01",
        "02-04-96"
      ),
      some.more.dates = c(
        "01 03 2015",
        "2nd January 2010",
        "01/05/1990",
        "03-Dec-2012",
        "02 April 2020"
      )
    )
    knitr::kable(bad.dates)

id | some.dates | some.more.dates
--- | --- | ---
1 | 02/05/92 | 01 03 2015
2 | 01-04-2020 | 2nd January 2010
3 | 1996/05/01 | 01/05/1990
4 | 2020-05-01 | 03-Dec-2012
5 | 02-04-96 | 02 April 2020

`fix_date_df()` requires two arguments, `df`, a data frame (or tibble) object and `col.names`, a character vector containing the names of columns to be standardized. By default, the first column of the data frame is assumed to contain row IDs. These IDs are used if a warning or error is raised to assist the user with locating the source of the error. The ID column can also be manually provided via the `id` argument.

The output from this function is a data frame or tibble (dependent on the object type of the the first argument, `df`) with the selected date columns now standardized and belonging to the `Date` class.

    fixed.dates <- fix_date_df(
      bad.dates,
      c("some.dates", "some.more.dates")
    )
    knitr::kable(fixed.dates)

id | some.dates | some.more.dates
--- | --- | ---
1 | 1992-05-02 | 2015-03-01
2 | 2020-04-01 | 2010-01-02
3 | 1996-05-01 | 1990-05-01
4 | 2020-05-01 | 2012-12-03
5 | 1996-04-02 | 2020-04-02

`datefixR` can handle many different formats including -, /, ., or white space separation, year-first or day-first, and month supplied as a number, an abbreviation or full length name.

`fix_date_char()` is similar to `fix_date_df()` but only applies to a single character object.

    fix_date_char("01 02 2014")
    #> [1] "2014-02-01"

# Advanced Topics

## Localization

`datefixR` currently supports dates being provided in English, Français (French), Deutsch (German), español (Spanish), and Русский (Russian) by recognizing both names of months in these languages and formatting customs. Expected languages do not need to be specified and can be provided just like any other date to be standardized.

    fix_date_char("7 de septiembre del 2014")
    #> [1] "2014-09-07"

Functions in `datefixR` assume day-first instead of month-first when day, month, and year are all given numerically (unless year is given first). However, this behavior can be modified by passing `format = "mdy"` to function calls.

    fix_date_char("01 02 2014", format = "mdy")
    #> [1] "2014-01-02"

If the month is given by name, then `datefixR` will automatically detect the correct format without the `format` argument needing to be specified by the user.

    fix_date_char("July 4th, 1776")
    #> [1] "1776-07-04"

## Date and Month Imputation

By default, `datefixR` imputes missing months as July, and missing days of the month as the first day. As such, “1992” converts to

    fix_date_char("1992")
    #> [1] "1992-07-01"

The argument for defaulting to July is 1st-2nd July is halfway through the year (on a non leap year). Therefore, assuming the year supplied is indeed correct, the imputation has a maximum potential error of 6 months from the true date. However, this behavior can be changed by supplying the `day.impute` and `month.impute` arguments with an integer corresponding to the desired day and month. For example, `day.impute = 1` and `month.impute = 1` results in the first day of January being imputed instead which is often a more common imputation for missing date data.

    fix_date_char("1992", day.impute = 1, month.impute = 1)
    #> [1] "1992-01-01"

The imputation mechanism can also be modified to impute `NA` if a month or day is missing by setting `day.impute` or `month.impute` to `NA`. This will also result in a warning being raised.

    fix_date_char("1992", month.impute = NA)
    #> [1] NA

Finally, imputation can be prevented by setting `day.impute` or `month.impute` to `NULL`. This will result in an error being raised if the day or month are missing respectively.

    fix_date_char("1992", month.impute = NULL)
    # ERROR

`day.impute` and `month.impute` can also be passed to `fix_date_df()` for similar functionality.

    example.df <- data.frame(
      id = seq(1, 3),
      some.dates = c("2014", "April 1990", "Mar 19")
    )
    fix_date_df(example.df, "some.dates", day.impute = 1, month.impute = 1)
    #>   id some.dates
    #> 1  1 2014-01-01
    #> 2  2 1990-04-01
    #> 3  3 2019-03-01

## Converting Numeric Dates

By default, if a date is given numerically (I.E no separators such as “/”, “-”, or white space) and is more than four character long, then this date is assumed to have been converted from `R`’s numeric date format. If a `Date` object is converted to a `numeric` object in R, the assigned value becomes the number of days from `1970-01-01`. Note that the date must still be converted to a `character` object before being passed to a `datefixR` function.

    date <- as.numeric(as.Date("2023-01-17"))
    print(date)
    #> [1] 19374
    fix_date_char(as.character(date))
    #> [1] "2023-01-17"

However if a date is converted to a numeric date in Excel, the number of days is instead counted from `1900-01-01`. If a user expects that dates to have been converted to numerical dates in Excel, then `excel = TRUE` can be passed to a `datefixR` function to rectify this.

    fix_date_char("44941", excel = TRUE)
    #> [1] "2023-01-15"

## Roman Numeral Months

Oracle Database can use Roman numerals to format months and this custom is also used in some biological contexts. If dates in need of parsing are in this format, `roman.numeral = TRUE` can be passed to `fix_date_char()` or `fix_date_df()`. This implementation is currently experimental and is not guaranteed to work alongside other date formats.

    fix_date_char("12/IV/2019", roman.numeral = TRUE)
    #> [1] "2019-04-12"

# Error & Edge-Case Handling

`datefixR` provides detailed error messages when it encounters dates which cannot be parsed. These errors often guide you to correct format issues or identify unsupported cases.

### Common Error Examples

- Unsupported Date Parts: If month names or formats aren’t recognized.
- Invalid Date Formats: Entering purely numeric values with unsupported separators.

    tryCatch(
      {
        fix_date_char("99-99-9999")
      },
      error = function(e) {
        cat("Error:", e$message, "\n")
      }
    )
    #> Error: Month not in expected range
    #>  for subject 1 (date: 99-99-9999)

# FAQ

## Understanding Year Parsing Logic

### Two-Digit vs Four-Digit Years

`datefixR` implements intelligent two-digit year expansion using a sliding window approach. The algorithm examines the first digit of a two-digit year and compares it to the third digit of the current year.

Algorithm Details: - If the first digit ≤ current year’s third digit: prefix with “20” - If the first digit > current year’s third digit: prefix with “19”

    # Current year: 2025 (third digit is 2)
    # Years 00-25 become 2000-2025
    fix_date_char("01/01/05") # → 2005-01-01
    fix_date_char("01/01/24") # → 2024-01-01

    # Years 26-99 become 1926-1999
    fix_date_char("06/15/92") # → 1992-06-15
    fix_date_char("03/10/80") # → 1980-03-10

Edge Case Behavior: As time progresses, this window shifts naturally. In 2030, years 00-30 will map to 2000-2030, while 31-99 map to 1931-1999.

    # Demonstrating current behavior (as of 2025)
    samples <- c("01/01/20", "01/01/24", "01/01/23", "01/01/50", "01/01/99")
    for (date in samples) {
      result <- fix_date_char(date)
      cat(sprintf("%s → %s\n", date, result))
    }
    #> 01/01/20 → 2020-01-01
    #> 01/01/24 → 2024-01-01
    #> 01/01/23 → 2023-01-01
    #> 01/01/50 → 1950-01-01
    #> 01/01/99 → 1999-01-01

## Automatic YMD Detection Logic

`datefixR` uses a sophisticated hierarchy to determine date component order:

### 1. Year-First Detection

If the first component is 4 digits, assumes `YYYY-MM-DD` format:

    fix_date_char("2023/12/25") # Automatically detects year-first
    #> [1] "2023-12-25"
    fix_date_char("1995-04-15") # Year-first with different separator
    #> [1] "1995-04-15"

### 2. Month Name Detection

If the first component is a month name, switches to `month-day-year` format:

    fix_date_char("January 15, 2023") # Month name → MDY
    #> [1] "2023-01-15"
    fix_date_char("Mar 5 1992") # Abbreviated month → MDY
    #> [1] "1992-03-05"
    fix_date_char("abril 20 2020") # Spanish month → MDY
    #> [1] "2020-04-20"

### 3. Numeric Component Defaults

For purely numeric dates, defaults to `day-month-year` unless overridden:

    # Default behavior (DMY)
    fix_date_char("15/03/2023") # → 2023-03-15 (day/month/year)
    #> [1] "2023-03-15"

    # Override with format parameter
    # fix_date_char("15/03/2023", format = "mdy") # → Invalid (month 15)
    fix_date_char("03/15/2023", format = "mdy") # → 2023-03-15 (month/day/year)
    #> [1] "2023-03-15"

### 4. Handling Ambiguous Cases

`datefixR` cannot resolve truly ambiguous dates without explicit format specification:

    # Ambiguous: could be March 5th or May 3rd
    fix_date_char("03/05/2023") # → 2023-05-03 (assumes DMY)
    #> [1] "2023-05-03"
    fix_date_char("03/05/2023", format = "mdy") # → 2023-03-05 (forces MDY)
    #> [1] "2023-03-05"

    # Unambiguous: day > 12 forces correct interpretation
    fix_date_char("15/03/2023") # → 2023-03-15 (only valid as DMY)
    #> [1] "2023-03-15"
    fix_date_char("03/15/2023", format = "mdy") # → 2023-03-15 (only valid as MDY)
    #> [1] "2023-03-15"

### 5. Format Detection Examples

Here’s how different inputs trigger specific detection logic:

    # Year-first detection (4-digit first component)
    test_dates_ymd <- c("2023/01/15", "1999-12-31", "2020.06.30")
    for (date in test_dates_ymd) {
      cat(sprintf("%s → %s (YMD detected)\n", date, fix_date_char(date)))
    }
    #> 2023/01/15 → 2023-01-15 (YMD detected)
    #> 1999-12-31 → 1999-12-31 (YMD detected)
    #> 2020.06.30 → 2020-06-30 (YMD detected)

    # Month-name detection (text month triggers MDY)
    test_dates_mdy <- c("March 15, 2023", "Dec 25 2020", "Jan 1st 2000")
    for (date in test_dates_mdy) {
      cat(sprintf("%s → %s (MDY detected)\n", date, fix_date_char(date)))
    }
    #> March 15, 2023 → 2023-03-15 (MDY detected)
    #> Dec 25 2020 → 2020-12-25 (MDY detected)
    #> Jan 1st 2000 → 2000-01-01 (MDY detected)

    # Default numeric (assumes DMY)
    test_dates_dmy <- c("15/03/2023", "01-12-1999", "25.12.2020")
    for (date in test_dates_dmy) {
      cat(sprintf("%s → %s (DMY default)\n", date, fix_date_char(date)))
    }
    #> 15/03/2023 → 2023-03-15 (DMY default)
    #> 01-12-1999 → 1999-12-01 (DMY default)
    #> 25.12.2020 → 2020-12-25 (DMY default)

## Overriding Automatic Detection

### Explicit Format Specification

When automatic detection fails or produces unwanted results, use the `format` parameter:

    # Force MDY interpretation
    fix_date_char("01/02/2023", format = "mdy") # → 2023-01-02 (Jan 2nd)
    #> [1] "2023-01-02"
    fix_date_char("01/02/2023", format = "dmy") # → 2023-02-01 (Feb 1st)
    #> [1] "2023-02-01"

    # Useful for consistently formatted datasets
    dates_usa <- c("01/15/2023", "03/22/2023", "12/01/2023")
    lapply(dates_usa, function(x) fix_date_char(x, format = "mdy"))
    #> [[1]]
    #> [1] "2023-01-15"
    #>
    #> [[2]]
    #> [1] "2023-03-22"
    #>
    #> [[3]]
    #> [1] "2023-12-01"

### Data Frame Processing

The same logic applies to `fix_date_df()` with consistent format specification:

    usa_dates <- data.frame(
      id = 1:3,
      event_date = c("01/15/2023", "03/22/2023", "12/01/2023")
    )

    # Apply consistent MDY format
    fixed_usa <- fix_date_df(usa_dates, "event_date", format = "mdy")
    knitr::kable(fixed_usa)

id | event_date
--- | ---
1 | 2023-01-15
2 | 2023-03-22
3 | 2023-12-01

## Citation

If you use this package in your research, please consider citing `datefixR`. An up-to-date citation can be obtained by running

    citation("datefixR")
    #> To cite package 'datefixR' in publications use:
    #>
    #>   Constantine-Cooke N (2025). _datefixR: Standardize Dates in Different
    #>   Formats or with Missing Data_. doi:10.5281/zenodo.5655311
    #>   <https://doi.org/10.5281/zenodo.5655311>. R package version 2.0.1,
    #>   <https://CRAN.R-project.org/package=datefixR>.
    #>
    #> A BibTeX entry for LaTeX users is
    #>
    #>   @Manual{,
    #>     title = {{datefixR}: Standardize Dates in Different Formats or with Missing Data},
    #>     author = {Nathan Constantine-Cooke},
    #>     year = {2025},
    #>     note = {R package version 2.0.1},
    #>     doi = {10.5281/zenodo.5655311},
    #>     url = {https://CRAN.R-project.org/package=datefixR},
    #>   }