Gracefully handle situation where TAF group end date comes before start date

Author: RISCfuture

Sources/SwiftMETAR/TAF/Parsers/PeriodParser.swift

@@ -74,6 +74,12 @@ class PeriodParser {
             let start = try periodStart.parse(match: match, referenceDate: referenceDate, originalString: periodStr),
                 end = try periodEnd.parse(match: match, referenceDate: referenceDate, afterDate: start, originalString: periodStr)
 
+            guard let startDate = start.date,
+                  let endDate = end.date,
+                  endDate >= startDate else {
+                throw Error.invalidPeriod(periodStr)
+            }
+
             return .init(start: start, end: end)
         }
     }

Tests/SwiftMETARTests/TAF/PeriodSpec.swift

@@ -22,5 +22,25 @@ class PeriodSpec: AsyncSpec {
             expect(period.end.month).to(equal(4))
             expect(period.end.day).to(equal(1))
         }
+
+        it("throws error for malformed BECMG with end date before start date") {
+            let string = """
+            TAF COR SPJL 241715Z 2418/2518 32018KT 9999 BKN020 TX17/2419Z TN04/2510Z TEMPO 2418/2422 32022G35KT SCT020TCU SCT100 BECMG 2423/2224 23007KT FM250300 VRB02KT 9999 SCT020
+            """
+            let referenceDate = Calendar.current.date(from: .init(year: 2025, month: 8, day: 24, hour: 18, minute: 0, second: 0))
+
+            await expect {
+                try await TAF.from(string: string, on: referenceDate)
+            }.to(throwError { error in
+                guard let swiftMETARError = error as? SwiftMETAR.Error else {
+                    fail("Expected SwiftMETAR.Error but got \(type(of: error))")
+                    return
+                }
+                guard case .invalidPeriod = swiftMETARError else {
+                    fail("Expected .invalidPeriod error but got \(swiftMETARError)")
+                    return
+                }
+            })
+        }
     }
 }