Date standardization

1. Date standardization

Date handling is a challenging aspect of data processing.

2. Why date standardization matters

We've extracted and modified the Product Name and Date Received columns from our grocery inventory dataset. But inconsistent date formats (slashes, dashes, or dots) make sorting and time-based analysis unreliable. Standardizing dates resolves these issues.

3. Converting string dates to LocalDate

Let's convert string dates using the `java.time.LocalDate` and `java.time.format.DateTimeFormatter` packages. `DateTimeFormatter` helps specify the pattern of our input date string. Here we expect `M/d/yy` format - `M` for month, `d` for day, and `yy` for two-digit year. When we parse the date with `LocalDate.parse()` using our formatter, Java converts it to a consistent format.

4. Standardizing different date formats

To handle multiple date formats, we create formatters for each pattern using `DateTimeFormatter.ofPattern()`. Then we parse dates with `LocalDate.parse()`, applying the matching formatter. While files typically use one pattern, this demonstrates handling multiple formats when needed.

5. Standardizing different date formats: outputs

Now all dates follow the same standard format, making comparisons and sorting operations straightforward.

6. Formatting dates for display

After standardizing dates, we can reformat them for reports or user interfaces. We start with a standardized `LocalDate` and use `DateTimeFormatter.ofPattern()` to specify our desired output format. Our pattern tells Java to display the full month name (MMMM), the day without leading zeros (d), and the full year (yyyy). After applying `date.format()`, the date becomes March 1, 2025.

7. Working with time zones

When working across time zones, let's convert an ISO date "2025-03-01" to specific zones. We parse it with `LocalDate.parse()`, then convert to New York time using `.atStartOfDay()` with `ZoneId.of("America/New_York")`. For Los Angeles, we use `.withZoneSameInstant()` to preserve the exact same moment - not create a new midnight in LA. This shows us that midnight in New York, which is 5 hours behind UTC, becomes 9 PM the previous day in Los Angeles, 8 hours behind UTC.

8. Why string manipulation fails with dates

String manipulation of dates leads to errors. For instance, checking if dates start with "3" to find March inventory works for "3/1/25" but fails for "03-15-25". Instead, it's better to parse dates into `LocalDate` format, then extract the month with `.getMonth()` and year with `.getYear()`.

9. Putting it all together

Java's date handling starts with key classes from the `java.time` package. To standardize dates, we first specify the input format using `DateTimeFormatter.ofPattern()`, then convert strings to `LocalDate` objects using `.parse()`. We create and convert time zones with `ZoneId.of()` and `.withZoneSameInstant()`. For output, we use `.format()` to display dates in the desired pattern. We should always use proper date parsing with `LocalDate.parse()` rather than unreliable string manipulation methods like `.startsWith()`. These tools ensure accurate date handling throughout our applications.

10. Let's practice!

Now you can practice standardizing dates!

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