Add ability to change timezone for calculating next run date (TheCodedSelf#28)

* Add ability to change timezone for calculating next run date

* Add unit tests for timezone implementation

* Discuss usage of timezone implementation in README

* Fix typo in README

Author: Wilsonator5000

Example/SwiftCronExample.xcodeproj/project.pbxproj

@@ -24,6 +24,7 @@
 		959A1B451D944A2700E685B3 /* MonthFieldTests.swift in Sources */ = {isa = PBXBuildFile; fileRef = 959A1B441D944A2700E685B3 /* MonthFieldTests.swift */; };
 		959A1B481D944CE400E685B3 /* DateBuilder.swift in Sources */ = {isa = PBXBuildFile; fileRef = 959A1B471D944CE400E685B3 /* DateBuilder.swift */; };
 		959A1B4D1D94518B00E685B3 /* TestData.swift in Sources */ = {isa = PBXBuildFile; fileRef = 959A1B4C1D94518B00E685B3 /* TestData.swift */; };
+		C8B4BFA221A067E800222752 /* TimeZoneTests.swift in Sources */ = {isa = PBXBuildFile; fileRef = C8B4BFA121A067E800222752 /* TimeZoneTests.swift */; };
 		E048D8BD8613C0CAF3BB4824 /* Pods_SwiftCronExample.framework in Frameworks */ = {isa = PBXBuildFile; fileRef = EB8E7C90A033C373325A9FC6 /* Pods_SwiftCronExample.framework */; };
 		FDCC4D322535804B48C02ECA /* Pods_SwiftCronExampleTests.framework in Frameworks */ = {isa = PBXBuildFile; fileRef = 9A585942BE5510532D56F9F1 /* Pods_SwiftCronExampleTests.framework */; };
 /* End PBXBuildFile section */
@@ -68,6 +69,7 @@
 		9A585942BE5510532D56F9F1 /* Pods_SwiftCronExampleTests.framework */ = {isa = PBXFileReference; explicitFileType = wrapper.framework; includeInIndex = 0; path = Pods_SwiftCronExampleTests.framework; sourceTree = BUILT_PRODUCTS_DIR; };
 		9FC712B82D2D075E149E73AB /* Pods-SwiftCronExampleTests.release.xcconfig */ = {isa = PBXFileReference; includeInIndex = 1; lastKnownFileType = text.xcconfig; name = "Pods-SwiftCronExampleTests.release.xcconfig"; path = "Pods/Target Support Files/Pods-SwiftCronExampleTests/Pods-SwiftCronExampleTests.release.xcconfig"; sourceTree = "<group>"; };
 		A8CB7B09FFB21F86177ED344 /* Pods-SwiftCronExample.release.xcconfig */ = {isa = PBXFileReference; includeInIndex = 1; lastKnownFileType = text.xcconfig; name = "Pods-SwiftCronExample.release.xcconfig"; path = "Pods/Target Support Files/Pods-SwiftCronExample/Pods-SwiftCronExample.release.xcconfig"; sourceTree = "<group>"; };
+		C8B4BFA121A067E800222752 /* TimeZoneTests.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = TimeZoneTests.swift; sourceTree = "<group>"; };
 		EB8E7C90A033C373325A9FC6 /* Pods_SwiftCronExample.framework */ = {isa = PBXFileReference; explicitFileType = wrapper.framework; includeInIndex = 0; path = Pods_SwiftCronExample.framework; sourceTree = BUILT_PRODUCTS_DIR; };
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@@ -163,6 +165,7 @@
 				8679D2A61CE1F5130071D783 /* SwiftCronExampleTests-Bridging-Header.h */,
 				8679D2B71CE1FA760071D783 /* DescriptionTests.swift */,
 				9529D3571DD37D91005ACCE0 /* CronExpressionTests.swift */,
+				C8B4BFA121A067E800222752 /* TimeZoneTests.swift */,
 			);
 			path = SwiftCronExampleTests;
 			sourceTree = "<group>";
@@ -401,6 +404,7 @@
 				959A1B4D1D94518B00E685B3 /* TestData.swift in Sources */,
 				8679D2B21CE1F51F0071D783 /* MonthTests.swift in Sources */,
 				8679D2B51CE1F51F0071D783 /* WeekdayTests.swift in Sources */,
+				C8B4BFA221A067E800222752 /* TimeZoneTests.swift in Sources */,
 				8679D2B11CE1F51F0071D783 /* MinuteTests.swift in Sources */,
 				9529D3581DD37D91005ACCE0 /* CronExpressionTests.swift in Sources */,
 				8679D2B31CE1F51F0071D783 /* SwiftCronTests.swift in Sources */,

Example/SwiftCronExampleTests/TimeZoneTests.swift

@@ -0,0 +1,27 @@
+import XCTest
+import SwiftCron
+
+class TimeZoneTests: XCTestCase {
+
+    override func setUp() {
+        super.setUp()
+        // Put setup code here. This method is called before the invocation of each test method in the class.
+    }
+
+    override func tearDown() {
+        // Put teardown code here. This method is called after the invocation of each test method in the class.
+        super.tearDown()
+    }
+
+    func testTimeZoneDifferencesResultInVariationOfNextRunDate() {
+        let newYorkTZ = TimeZone(identifier: "America/New_York")!
+        let losAngelesTZ = TimeZone(identifier: "America/Los_Angeles")!
+
+        let cronExpression = CronExpression(cronString: "0 10 12 9 * *")!
+
+        let nextRunNewYorkTZ = cronExpression.getNextRunDateFromNow(adjustingForTimeZone: newYorkTZ)!
+        let nextRunLosAngelesTZ = cronExpression.getNextRunDateFromNow(adjustingForTimeZone: losAngelesTZ)!
+
+        XCTAssertNotEqual(nextRunNewYorkTZ, nextRunLosAngelesTZ)
+    }
+}

README.md

@@ -84,6 +84,20 @@ let dateToStartSearchOn = NSDate()
 let nextRunDate = myCronExpression.getNextRunDate(dateToStartSearchOn)
 ```
 
+##### Custom timezones
+
+By default SwiftCron uses the system's current timezone to calculate the next run date. To override this behavior, just pass the timezone you want into `CronExpression.getNextRunDateFromNow(adjustingForTimeZone:)` or `CronExpression.getNextRunDate(_:adjustingForTimeZone:)` methods.
+
+```swift
+// Every 12th September at 10 AM
+let cronExpression = CronExpression(cronString: "0 10 12 9 * *")
+
+// Calculate the next run date as if the cron expression was created in the
+// America/Los_Angeles timezone
+let losAngeles = TimeZone(identifier: "America/Los_Angeles")!
+let nextRun = cronExpression.getNextRunDateFromNow(adjustingForTimeZone: losAngeles)
+```
+
 ## Contributing
 
 - Pull requests for bug fixes and new features are most welcome.

Sources/CronExpression.swift

@@ -62,13 +62,22 @@ public class CronExpression {
 
 	// MARK: - Get Next Run Date
 
-	public func getNextRunDateFromNow() -> Date? {
-		return getNextRunDate(Date())
+    public func getNextRunDateFromNow(adjustingForTimeZone outputTimeZone: TimeZone = .current) -> Date? {
+		return getNextRunDate(Date(), adjustingForTimeZone: outputTimeZone)
 	}
 
-	public func getNextRunDate(_ date: Date) -> Date? {
-		return getNextRunDate(date, skip: 0)
+	public func getNextRunDate(_ date: Date, adjustingForTimeZone outputTimeZone: TimeZone = .current) -> Date? {
+        guard let nextRun = getNextRunDate(date, skip: 0) else { return nil }
+        return adjust(date: nextRun, for: outputTimeZone)
 	}
+    
+    func adjust(date: Date, for timeZone: TimeZone) -> Date {
+        let currentTZOffset = TimeZone.current.secondsFromGMT(for: date)
+        let outputTZOffset = timeZone.secondsFromGMT(for: date)
+        
+        let tzDifference = TimeInterval(currentTZOffset - outputTZOffset)
+        return date.addingTimeInterval(tzDifference)
+    }
 
 	func getNextRunDate(_ date: Date, skip: Int) -> Date? {
 		guard matchIsTheoreticallyPossible(date) else {