diff --git a/projects/core/src/main/java/dan200/computercraft/core/apis/LuaDateTime.java b/projects/core/src/main/java/dan200/computercraft/core/apis/LuaDateTime.java
index a90f41ef1..d9e987129 100644
--- a/projects/core/src/main/java/dan200/computercraft/core/apis/LuaDateTime.java
+++ b/projects/core/src/main/java/dan200/computercraft/core/apis/LuaDateTime.java
@@ -47,7 +47,7 @@ static void format(DateTimeFormatterBuilder formatter, String format) throws Lua
                         case 'g' -> formatter.appendValueReduced(IsoFields.WEEK_BASED_YEAR, 2, 2, 0);
                         case 'G' -> formatter.appendValue(IsoFields.WEEK_BASED_YEAR);
                         case 'H' -> formatter.appendValue(ChronoField.HOUR_OF_DAY, 2);
-                        case 'I' -> formatter.appendValue(ChronoField.HOUR_OF_AMPM, 2);
+                        case 'I' -> formatter.appendValue(ChronoField.CLOCK_HOUR_OF_AMPM, 2);
                         case 'j' -> formatter.appendValue(ChronoField.DAY_OF_YEAR, 3);
                         case 'm' -> formatter.appendValue(ChronoField.MONTH_OF_YEAR, 2);
                         case 'M' -> formatter.appendValue(ChronoField.MINUTE_OF_HOUR, 2);
diff --git a/projects/core/src/test/resources/test-rom/spec/apis/os_spec.lua b/projects/core/src/test/resources/test-rom/spec/apis/os_spec.lua
index a9a620d50..12aaadced 100644
--- a/projects/core/src/test/resources/test-rom/spec/apis/os_spec.lua
+++ b/projects/core/src/test/resources/test-rom/spec/apis/os_spec.lua
@@ -96,6 +96,9 @@ describe("The os library", function()
             exp_code("%Y", "2000")
             exp_code("%%", "%")
 
+            it("%r at 12 AM", function() expect(os.date("%r", 1670373922)):eq("12:45:22 AM") end)
+            it("%I at 12 AM", function() expect(os.date("%I", 1670373922)):eq("12") end)
+
             it("zones are numbers", function()
                 local zone = os.date("%z", t1)
                 if not zone:match("^[+-]%d%d%d%d$") then