diff --git a/src/main.zig b/src/main.zig
index c5839ac..998c384 100644
--- a/src/main.zig
+++ b/src/main.zig
@@ -100,8 +100,26 @@ pub fn nanoToSecond(nano: u64) u64 {
     return sec;
 }
 
+pub fn fmtDigit(n: u8) [2]u8 {
+    if (n < 10) {
+        return [_]u8{ '0', '0' + n };
+    }
+    var t = n / 10; // 0*
+    var o = n % 10; // *0
+    return [_]u8{ '0' + t, '0' + o };
+}
+
+test "test fmtDigit" {
+    var dig: u64 = 8;
+    var result = fmtDigit(@intCast(dig));
+    if (!std.mem.eql(u8, &result, &[_]u8{ '0', '8' })) std.debug.panic("エラー：fmtDigit = {s}\n", .{result});
+    try testing.expect(std.mem.eql(u8, &result, &[_]u8{ '0', '8' }));
+}
+
 test "test nanoToSecond" {
-    try testing.expect(nanoToSecond(1690514984429000696) == 1690514984);
+    var result = nanoToSecond(1690514984429000696);
+    if (result != 1690514984) std.debug.panic("エラー：nanoToSecond = {}\n", .{result});
+    try testing.expect(result == 1690514984);
 }
 
 test "test unixToDateTime" {