feat(version): add Unix epoch support for buildTime

Support both Unix epoch timestamps and RFC3339 format for build time
injection via ldflags. Unix epoch format provides simpler build
commands: $(date +%s) vs $(date -u +%Y-%m-%dT%H:%M:%SZ).

- Add parseBuildTime() to convert epoch to RFC3339
- Maintain backward compatibility with existing RFC3339 format
- Ensure consistent RFC3339 output regardless of input format
- Fix build date priority over git commit time

version/version_test.go

@@ -309,3 +309,106 @@ func BenchmarkCheckVersionDevSnapshot(b *testing.B) {
 		_ = CheckVersion(version, minimum)
 	}
 }
+
+func TestParseBuildTime(t *testing.T) {
+	tests := []struct {
+		name     string
+		input    string
+		expected string
+	}{
+		{
+			name:     "Unix epoch timestamp",
+			input:    "1672531200", // 2023-01-01T00:00:00Z
+			expected: "2023-01-01T00:00:00Z",
+		},
+		{
+			name:     "Unix epoch zero",
+			input:    "0",
+			expected: "1970-01-01T00:00:00Z",
+		},
+		{
+			name:     "Valid RFC3339 format",
+			input:    "2023-12-25T15:30:45Z",
+			expected: "2023-12-25T15:30:45Z",
+		},
+		{
+			name:     "RFC3339 with timezone",
+			input:    "2023-12-25T10:30:45-05:00",
+			expected: "2023-12-25T10:30:45-05:00",
+		},
+		{
+			name:     "Empty string",
+			input:    "",
+			expected: "",
+		},
+		{
+			name:     "Invalid format - return unchanged",
+			input:    "not-a-date",
+			expected: "not-a-date",
+		},
+		{
+			name:     "Invalid timestamp - return unchanged",
+			input:    "invalid-timestamp",
+			expected: "invalid-timestamp",
+		},
+		{
+			name:     "Partial date - return unchanged",
+			input:    "2023-01-01",
+			expected: "2023-01-01",
+		},
+		{
+			name:     "Negative epoch - should work",
+			input:    "-1",
+			expected: "1969-12-31T23:59:59Z",
+		},
+		{
+			name:     "Large epoch timestamp",
+			input:    "4102444800", // 2100-01-01T00:00:00Z
+			expected: "2100-01-01T00:00:00Z",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			result := parseBuildTime(tt.input)
+			if result != tt.expected {
+				t.Errorf("parseBuildTime(%q) = %q, expected %q", tt.input, result, tt.expected)
+			}
+		})
+	}
+}
+
+func TestParseBuildTimeConsistency(t *testing.T) {
+	// Test that calling parseBuildTime multiple times with the same input returns the same result
+	testInputs := []string{
+		"1672531200",
+		"2023-01-01T00:00:00Z",
+		"invalid-date",
+		"",
+	}
+
+	for _, input := range testInputs {
+		result1 := parseBuildTime(input)
+		result2 := parseBuildTime(input)
+		if result1 != result2 {
+			t.Errorf("parseBuildTime(%q) not consistent: %q != %q", input, result1, result2)
+		}
+	}
+}
+
+func BenchmarkParseBuildTime(b *testing.B) {
+	inputs := []string{
+		"1672531200",                 // Unix epoch
+		"2023-01-01T00:00:00Z",      // RFC3339
+		"invalid-timestamp",          // Invalid
+		"",                          // Empty
+	}
+
+	for _, input := range inputs {
+		b.Run(input, func(b *testing.B) {
+			for i := 0; i < b.N; i++ {
+				_ = parseBuildTime(input)
+			}
+		})
+	}
+}