How to subtract X days from a date using Java calendar?

Dates and time zones

Different time zones in Java can transform your midnight into a bright morning elsewhere. The TimeZone class comes handy to adjust accordingly:

TimeZone tz = TimeZone.getTimeZone("America/New_York"); // Destination timezone
Calendar cal = Calendar.getInstance(tz); // Request time-travel coffee here
cal.add(Calendar.DAY_OF_MONTH, -X); // Discussing time leap specifics
Date newDate = cal.getTime(); // Voila, your cup of Joe across time zones
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Subtract days using Java 8

The java.time package (Java 8 and onwards) brings user-friendly date operations:

LocalDate now = LocalDate.now(); // Current date
LocalDate newDate = now.minusDays(X); // Subtract days using java.time
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It's clearer and concise with self-explanatory function names.

What about immutability?

java.time classes, including LocalDate, are immutable - they don't change their state! So, minusDays returns a new instance, not altering the original one:

LocalDate now = LocalDate.now(); // Current date
LocalDate future = now.minusDays(X); // Subtract days - now remains untouched!
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Joda-Time for rescue

For more complex date-time operations or stuck with pre-Java 8 code? Joda-Time library is your good old friend:

DateTime now = new DateTime(); // Joda-Time variant of "now"
DateTime newDate = now.minusDays(X); // Subtracting days like a boss
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Joda-Time's DateTime offers a simple and effective medium to handle dates.

The twilight of daylight saving

The transition of Daylight Saving Time (DST) can be a tricky one when subtracting dates. A 24-hour day might deviate. Validate the date and time after the subtraction process.

Beware of millisecond conversion

Direct subtraction by converting days into milliseconds may look appealing but isn't recommended due to subtleties like leap seconds and DST:

long millis = System.currentTimeMillis(); // The now in milliseconds
long xDaysInMillis = X * 24 * 60 * 60 * 1000;  // Quite a math, isn't it?
Date newDate = new Date(millis - xDaysInMillis); // Not as simple as it looks!
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It's less precise and error-prone.

Old school: Date API

Dealing with a Jurassic park? Here, Calendar acts as a bridge to manipulate Date in pre-Java 8 applications:

Date now = new Date(); // old but gold
Calendar cal = Calendar.getInstance(); // Your reliable time butler
cal.setTime(now);
cal.add(Calendar.DAY_OF_MONTH, -X); // Calendar, do the math!
Date newDate = cal.getTime(); // Reliable but less intuitive result
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