diff --git a/icu4c/source/i18n/udat.cpp b/icu4c/source/i18n/udat.cpp
index ab91bcff412..897ac26c01d 100644
--- a/icu4c/source/i18n/udat.cpp
+++ b/icu4c/source/i18n/udat.cpp
@@ -82,13 +82,18 @@ static UCalendarDateFields gDateFieldMapping[] = {
     UCAL_ZONE_OFFSET,          // UDAT_TIMEZONE_ISO_FIELD = 32 (also UCAL_DST_OFFSET)
     UCAL_ZONE_OFFSET,          // UDAT_TIMEZONE_ISO_LOCAL_FIELD = 33 (also UCAL_DST_OFFSET)
     UCAL_EXTENDED_YEAR,        // UDAT_RELATED_YEAR_FIELD = 34 (not an exact match)
-    UCAL_FIELD_COUNT,          // UDAT_FIELD_COUNT = 35
+    UCAL_FIELD_COUNT,          // UDAT_AM_PM_MIDNIGHT_NOON_FIELD=35 (no match)
+    UCAL_FIELD_COUNT,          // UDAT_FLEXIBLE_DAY_PERIOD_FIELD=36 (no match)
+    UCAL_FIELD_COUNT,          // UDAT_TIME_SEPARATOR_FIELD = 37 (no match)
+                               // UDAT_FIELD_COUNT = 38 as of ICU 67
     // UCAL_IS_LEAP_MONTH is not the target of a mapping
 };
 
 U_CAPI UCalendarDateFields U_EXPORT2
 udat_toCalendarDateField(UDateFormatField field) {
-  return gDateFieldMapping[field];
+  static_assert(UDAT_FIELD_COUNT == UPRV_LENGTHOF(gDateFieldMapping),
+    "UDateFormatField and gDateFieldMapping should have the same number of entries and be kept in sync.");
+  return (field >= UDAT_ERA_FIELD && field < UPRV_LENGTHOF(gDateFieldMapping))? gDateFieldMapping[field]: UCAL_FIELD_COUNT;
 }
 
 /* For now- one opener. */
diff --git a/icu4c/source/i18n/unicode/udat.h b/icu4c/source/i18n/unicode/udat.h
index cf7a165e70a..4610203ddca 100644
--- a/icu4c/source/i18n/unicode/udat.h
+++ b/icu4c/source/i18n/unicode/udat.h
@@ -832,10 +832,24 @@ typedef enum UDateFormatField {
 
 /**
  * Maps from a UDateFormatField to the corresponding UCalendarDateFields.
- * Note: since the mapping is many-to-one, there is no inverse mapping.
+ *
+ * Note 1: Since the mapping is many-to-one, there is no inverse mapping.
+ *
+ * Note 2: There is no UErrorCode parameter, so in case of error (UDateFormatField is
+ * unknown or has no corresponding UCalendarDateFields value), the function returns the
+ * current value of UCAL_FIELD_COUNT. However, that value may change from release to
+ * release and is consequently deprecated. For a future-proof runtime way of checking
+ * for errors:
+ * a) First save the value returned by the function when it is passed an invalid value
+ *    such as "(UDateFormatField)-1".
+ * b) Then, to test for errors when passing some other UDateFormatField value, check
+ *     whether the function returns that saved value.
+ *
  * @param field the UDateFormatField.
- * @return the UCalendarDateField.  This will be UCAL_FIELD_COUNT in case
- * of error (e.g., the input field is UDAT_FIELD_COUNT).
+ * @return the UCalendarDateField. In case of error (UDateFormatField is unknown or has
+ *   no corresponding UCalendarDateFields value) this will be the current value of
+ *   UCAL_FIELD_COUNT, but that value may change from release to release.
+ *   See Note 2 above.
  * @stable ICU 4.4
  */
 U_CAPI UCalendarDateFields U_EXPORT2
diff --git a/icu4c/source/test/cintltst/cdattst.c b/icu4c/source/test/cintltst/cdattst.c
index d5e3ba2ccd9..854eb3b6911 100644
--- a/icu4c/source/test/cintltst/cdattst.c
+++ b/icu4c/source/test/cintltst/cdattst.c
@@ -43,6 +43,7 @@ static void TestCalendarDateParse(void);
 static void TestParseErrorReturnValue(void);
 static void TestFormatForFields(void);
 static void TestForceGannenNumbering(void);
+static void TestMapDateToCalFields(void);
 
 void addDateForTest(TestNode** root);
 
@@ -63,6 +64,7 @@ void addDateForTest(TestNode** root)
     TESTCASE(TestParseErrorReturnValue);
     TESTCASE(TestFormatForFields);
     TESTCASE(TestForceGannenNumbering);
+    TESTCASE(TestMapDateToCalFields);
 }
 /* Testing the DateFormat API */
 static void TestDateFormat()
@@ -1905,4 +1907,35 @@ static void TestForceGannenNumbering(void) {
         udatpg_close(dtpgen);
     }
 }
+
+typedef struct {
+    UChar               patternChar; // for future use
+    UDateFormatField    dateField;
+    UCalendarDateFields calField;
+} PatternCharToFieldsItem;
+
+static const PatternCharToFieldsItem patCharToFieldsItems[] = {
+    { u'G', UDAT_ERA_FIELD,                 UCAL_ERA },
+    { u'y', UDAT_YEAR_FIELD,                UCAL_YEAR },
+    { u'Y', UDAT_YEAR_WOY_FIELD,            UCAL_YEAR_WOY },
+    { u'Q', UDAT_QUARTER_FIELD,             UCAL_MONTH },
+    { u'H', UDAT_HOUR_OF_DAY0_FIELD,        UCAL_HOUR_OF_DAY },
+    { u'r', UDAT_RELATED_YEAR_FIELD,        UCAL_EXTENDED_YEAR },
+    { u'B', UDAT_FLEXIBLE_DAY_PERIOD_FIELD, UCAL_FIELD_COUNT },
+    { u'$', UDAT_FIELD_COUNT,               UCAL_FIELD_COUNT },
+    { 0xFFFF, (UDateFormatField)-1,         UCAL_FIELD_COUNT }, // patternChar ignored here
+    { (UChar)0, (UDateFormatField)0, (UCalendarDateFields)0 } // terminator
+};
+
+static void TestMapDateToCalFields(void){
+    const PatternCharToFieldsItem* itemPtr;
+    for ( itemPtr=patCharToFieldsItems; itemPtr->patternChar!=(UChar)0; itemPtr++) {
+        UCalendarDateFields calField = udat_toCalendarDateField(itemPtr->dateField);
+        if (calField != itemPtr->calField) {
+            log_err("for pattern char 0x%04X, dateField %d, expect calField %d and got %d\n",
+                    itemPtr->patternChar, itemPtr->dateField, itemPtr->calField, calField);
+        }
+    }
+}
+
 #endif /* #if !UCONFIG_NO_FORMATTING */