Hey there! Let's break down why your current code isn't working and get those dates standardized to mm/dd/yyyy format properly.

The Problem with Your Current Code #

Your ActivityDay column has two different date formats: mm-dd-yyyy (like 04-12-2016) and mm/dd/yyyy (like 4/13/2016). Here's why your code failed:

• You used as.Date(..., format = "%m%d%Y") — this format string tells R to expect dates without any separators (e.g., 04122016), which doesn't match either of your actual formats.
• Even if you got the format string right, as.Date() can only handle one specific format at a time, so it wouldn't parse both - and / separators automatically.
• Also, R's Date object stores dates as numeric values (days since 1970-01-01), not as formatted strings. The default display might not be mm/dd/yyyy even after successful conversion.

The Solution Using lubridate #

Since you're already using lubridate (part of tidyverse), this is perfect — it's designed to handle mixed date formats seamlessly when the date order is consistent (month-day-year in your case).

Here's the corrected code:

# Load required libraries (no need to reinstall tidyverse every time!)
library(tidyverse)
library(lubridate)

# Import your data (keep this part as-is)
dailyCalories_merged <- read.csv("dailyCalories_merged.csv", header = TRUE, sep = ",")

# Step 1: Convert mixed character dates to R's Date type
# mdy() automatically recognizes month-day-year order, regardless of separator (- or /)
dailyCalories_merged$ActivityDay <- mdy(dailyCalories_merged$ActivityDay)

# Step 2: Format the Date object to mm/dd/yyyy as a character string (for display/output)
# Use this if you need the column to show as mm/dd/yyyy instead of R's default format
dailyCalories_merged$ActivityDay_formatted <- format(dailyCalories_merged$ActivityDay, "%m/%d/%Y")

What This Does #

1. mdy(): This function scans each date string, identifies it as month-day-year, and converts it to a proper Date object — it doesn't care if the separator is - or /.
2. format(): If you need the date to appear as mm/dd/yyyy (e.g., for exporting or reporting), this converts the Date object to a character string in your desired format. If you just need the date for calculations, you can skip this step since Date objects work fine for time-based operations.

Verify the Result #

Run this to check that your dates are now standardized:

head(dailyCalories_merged)

You should see ActivityDay as a consistent Date type (displayed based on your locale, but internally uniform) and ActivityDay_formatted as a string in mm/dd/yyyy format.

内容的提问来源于stack exchange，提问作者Prasad Jahagirdar