Add support for day count dates, RFC 8943 (#106)

This addresses #98 
diff --git a/inc/qcbor/qcbor_common.h b/inc/qcbor/qcbor_common.h
index 6883e1b..7e52a26 100644
--- a/inc/qcbor/qcbor_common.h
+++ b/inc/qcbor/qcbor_common.h
@@ -204,12 +204,20 @@
     (https://tools.ietf.org/html/rfc8152). No API is provided for this
     tag. */
 #define CBOR_TAG_SIGN          98
+/** Tag for date counted by days from Jan 1 1970 per [RFC 8943]
+    (https://tools.ietf.org/html/rfc8943). See
+    QCBOREncode_AddTDaysEpoch(). */
+#define CBOR_TAG_DAYS_EPOCH    100
 /** Not Decoded by QCBOR. World geographic coordinates. See ISO 6709, [RFC 5870]
     (https://tools.ietf.org/html/rfc5870) and WGS-84. No API is
     provided for this tag. */
 #define CBOR_TAG_GEO_COORD     103
 /** Binary MIME.*/
 #define CBOR_TAG_BINARY_MIME   257
+/** Tag for date string without time or time zone per [RFC 8943]
+    (https://tools.ietf.org/html/rfc8943). See
+    QCBOREncode_AddTDaysString(). */
+#define CBOR_TAG_DAYS_STRING   1004
 /** The magic number, self-described CBOR. No API is provided for this
     tag. */
 #define CBOR_TAG_CBOR_MAGIC    55799
diff --git a/inc/qcbor/qcbor_decode.h b/inc/qcbor/qcbor_decode.h
index 94126bd..a1a64b9 100644
--- a/inc/qcbor/qcbor_decode.h
+++ b/inc/qcbor/qcbor_decode.h
@@ -204,8 +204,6 @@
     being traversed as an array. See QCBORDecode_Init() */
 #define QCBOR_TYPE_MAP_AS_ARRAY  32
 
-/* Start of QCBOR types that are defined as the CBOR tag + 12 */
-
 /** Encoded CBOR that is wrapped in a byte string. Often used when the
     CBOR is to be hashed for signing or HMAC. See also @ref
     QBCOR_TYPE_WRAPPED_CBOR_SEQUENCE. Data is in @c val.string. */
@@ -236,12 +234,20 @@
     QBCOR_TYPE_WRAPPED_CBOR.  Data is in @c val.string. */
 #define QBCOR_TYPE_WRAPPED_CBOR_SEQUENCE  75
 
-/* End of QCBOR types that are CBOR tag + 12 */
-
 /** Binary MIME per RFC 2045. See also @ref QCBOR_TYPE_MIME. Data is
     in @c val.string. */
 #define QCBOR_TYPE_BINARY_MIME   76
 
+/** Type for [RFC 8943](https://tools.ietf.org/html/rfc8943) date
+    string, a date with no time or time zone info. Data is in
+    @c val.string */
+#define QCBOR_TYPE_DAYS_STRING   77
+
+/** Type for integer days since Jan 1 1970 described in
+    [RFC 8943](https://tools.ietf.org/html/rfc8943). Data is in
+    @c val.epochDays */
+#define QCBOR_TYPE_DAYS_EPOCH    78
+
 #define QCBOR_TYPE_TAG        254 // Used internally; never returned
 
 #define QCBOR_TYPE_OPTTAG   QCBOR_TYPE_TAG // Depricated in favor of QCBOR_TYPE_TAG
@@ -290,7 +296,8 @@
       /** The value for uDataType @ref QCBOR_TYPE_UINT64. */
       uint64_t    uint64;
       /** The value for @c uDataType @ref QCBOR_TYPE_BYTE_STRING and
-          @ref QCBOR_TYPE_TEXT_STRING. */
+          @ref QCBOR_TYPE_TEXT_STRING. Also
+          @ref QCBOR_TYPE_DAYS_STRING. */
       UsefulBufC  string;
       /** The "value" for @c uDataType @ref QCBOR_TYPE_ARRAY or @ref
           QCBOR_TYPE_MAP -- the number of items in the array or map.
@@ -311,8 +318,14 @@
          int64_t  nSeconds;
          double   fSecondsFraction;
       } epochDate;
+
+      /** The value for @c uDataType @ref QCBOR_TYPE_DAYS_EPOCH -- the
+          number of days before or after Jan 1, 1970. */
+      int64_t     epochDays;
+
       /** The value for @c uDataType @ref QCBOR_TYPE_DATE_STRING. */
       UsefulBufC  dateString;
+
       /** The value for @c uDataType @ref QCBOR_TYPE_POSBIGNUM and
            @ref QCBOR_TYPE_NEGBIGNUM. */
       UsefulBufC  bigNum;
diff --git a/inc/qcbor/qcbor_encode.h b/inc/qcbor/qcbor_encode.h
index ba654ee..016226b 100644
--- a/inc/qcbor/qcbor_encode.h
+++ b/inc/qcbor/qcbor_encode.h
@@ -730,7 +730,7 @@
  this is always UTC and does not include the time zone.  Use
  QCBOREncode_AddDateString() if you want to include the time zone.
 
- The integer encoding rules apply here so the date will be encoded in
+ The preferred integer encoding rules apply here so the date will be encoded in
  a minimal number of bytes. Until about the year 2106 these dates will
  encode in 6 bytes -- one byte for the tag, one byte for the type and
  4 bytes for the integer. After that it will encode to 10 bytes.
@@ -743,10 +743,11 @@
 
  This implementation cannot encode fractional seconds using float or
  double even though that is allowed by CBOR, but you can encode them
- if you want to by calling QCBOREncode_AddDouble() and
- QCBOREncode_AddTag().
+ if you want to by calling QCBOREncode_AddTag() and QCBOREncode_AddDouble().
 
  Error handling is the same as QCBOREncode_AddInt64().
+
+ See also QCBOREncode_AddTDaysEpoch().
  */
 static void QCBOREncode_AddTDateEpoch(QCBOREncodeContext *pCtx,
                                       uint8_t             uTagRequirement,
@@ -775,6 +776,43 @@
                                            int64_t             nDate);
 
 
+
+/**
+  @brief  Add an epoch-based day-count date.
+
+  @param[in] pCtx             The encoding context to add the date to.
+  @param[in] uTagRequirement  Either @ref QCBOR_ENCODE_AS_TAG or
+                              @ref QCBOR_ENCODE_AS_BORROWED.
+  @param[in] nDays            Number of days before or after 1970-01-0.
+
+ This date format is described in
+ [RFC 8943] (https://tools.ietf.org/html/rfc8943).
+
+ The integer encoding rules apply here so the date will be encoded in
+ a minimal number of bytes. Until about the year 2149 these dates will
+ encode in 4 bytes -- one byte for the tag, one byte for the type and
+ 2 bytes for the integer.
+
+ See also QCBOREncode_AddTDateEpoch().
+
+*/
+static void QCBOREncode_AddTDaysEpoch(QCBOREncodeContext *pCtx,
+                                      uint8_t             uTagRequirement,
+                                      int64_t             nDays);
+
+static void QCBOREncode_AddTDaysEpochToMapSZ(QCBOREncodeContext *pCtx,
+                                             const char         *szLabel,
+                                             uint8_t             uTagRequirement,
+                                             int64_t             nDays);
+
+static void QCBOREncode_AddTDaysEpochToMapN(QCBOREncodeContext *pCtx,
+                                            int64_t             nLabel,
+                                            uint8_t             uTagRequirement,
+                                            int64_t             nDays);
+
+
+
+
 /**
  @brief Add a byte string to the encoded output.
 
@@ -1403,6 +1441,8 @@
  CBOR will be incorrect and the receiver may not be able to handle it.
 
  Error handling is the same as QCBOREncode_AddInt64().
+
+ See also QCBOREncode_AddTDayString().
  */
 static void QCBOREncode_AddTDateString(QCBOREncodeContext *pCtx,
                                        uint8_t             uTagRequirement,
@@ -1430,6 +1470,50 @@
                                             int64_t             nLabel,
                                             const char         *szDate);
 
+
+/**
+ @brief  Add a date-only string.
+
+ @param[in] pCtx             The encoding context to add the date to.
+ @param[in] uTagRequirement  Either @ref QCBOR_ENCODE_AS_TAG or
+                             @ref QCBOR_ENCODE_AS_BORROWED.
+ @param[in] szDate           Null-terminated string with date to add.
+
+ This date format is described in
+ [RFC 8943] (https://tools.ietf.org/html/rfc8943), but that mainly
+ references RFC 3339.  The string szDate must be in the forrm
+ specified the ABNF for a full-date in
+ [RFC 3339] (https://tools.ietf.org/html/rfc3339). Examples of this
+ are "1985-04-12" and "1937-01-01".  The time and the time zone are
+ never included.
+
+ Note that this function doesn't validate the format of the date
+ string at all. If you add an incorrect format date string, the
+ generated CBOR will be incorrect and the receiver may not be able to
+ handle it.
+
+ Error handling is the same as QCBOREncode_AddInt64().
+
+ See also QCBOREncode_AddTDateString().
+ */
+static void
+QCBOREncode_AddTDaysString(QCBOREncodeContext *pCtx,
+                           uint8_t             uTagRequirement,
+                           const char         *szDate);
+
+static void
+QCBOREncode_AddTDaysStringToMapSZ(QCBOREncodeContext *pCtx,
+                                  const char         *szLabel,
+                                  uint8_t             uTagRequirement,
+                                  const char         *szDate);
+
+static void
+QCBOREncode_AddTDaysStringToMapN(QCBOREncodeContext *pCtx,
+                                 int64_t             nLabel,
+                                 uint8_t             uTagRequirement,
+                                 const char         *szDate);
+
+
 /**
  @brief  Add a standard Boolean.
 
@@ -2214,6 +2298,29 @@
 }
 
 
+static inline void
+QCBOREncode_AddTDaysEpoch(QCBOREncodeContext *pMe, uint8_t uTag, int64_t nDays)
+{
+   if(uTag == QCBOR_ENCODE_AS_TAG) {
+      QCBOREncode_AddTag(pMe, CBOR_TAG_DAYS_EPOCH);
+   }
+   QCBOREncode_AddInt64(pMe, nDays);
+}
+
+static inline void
+QCBOREncode_AddTDaysEpochToMapSZ(QCBOREncodeContext *pMe, const char *szLabel, uint8_t uTag, int64_t nDays)
+{
+   QCBOREncode_AddSZString(pMe, szLabel);
+   QCBOREncode_AddTDaysEpoch(pMe, uTag, nDays);
+}
+
+static inline void
+QCBOREncode_AddTDaysEpochToMapN(QCBOREncodeContext *pMe, int64_t nLabel, uint8_t uTag, int64_t nDays)
+{
+   QCBOREncode_AddInt64(pMe, nLabel);
+   QCBOREncode_AddTDaysEpoch(pMe, uTag, nDays);
+}
+
 
 static inline void
 QCBOREncode_AddBytes(QCBOREncodeContext *pMe, UsefulBufC Bytes)
@@ -2957,6 +3064,34 @@
 
 
 static inline void
+QCBOREncode_AddTDaysString(QCBOREncodeContext *pMe, uint8_t uTagRequirement, const char *szDate)
+{
+   if(uTagRequirement == QCBOR_ENCODE_AS_TAG) {
+      QCBOREncode_AddTag(pMe, CBOR_TAG_DAYS_STRING);
+   }
+   QCBOREncode_AddSZString(pMe, szDate);
+}
+
+static inline void
+QCBOREncode_AddTDaysStringToMapSZ(QCBOREncodeContext *pMe,
+                                  const char         *szLabel,
+                                  uint8_t             uTagRequirement,
+                                  const char         *szDate)
+{
+   QCBOREncode_AddSZString(pMe, szLabel);
+   QCBOREncode_AddTDaysString(pMe, uTagRequirement, szDate);
+}
+
+static inline void
+QCBOREncode_AddTDaysStringToMapN(QCBOREncodeContext *pMe, int64_t nLabel, uint8_t uTagRequirement, const char *szDate)
+{
+   QCBOREncode_AddInt64(pMe, nLabel);
+   QCBOREncode_AddTDaysString(pMe, uTagRequirement, szDate);
+}
+
+
+
+static inline void
 QCBOREncode_AddSimple(QCBOREncodeContext *pMe, uint64_t uNum)
 {
    QCBOREncode_AddType7(pMe, 0, uNum);
diff --git a/inc/qcbor/qcbor_spiffy_decode.h b/inc/qcbor/qcbor_spiffy_decode.h
index 3e69221..5c28c7e 100644
--- a/inc/qcbor/qcbor_spiffy_decode.h
+++ b/inc/qcbor/qcbor_spiffy_decode.h
@@ -991,7 +991,7 @@
 
  @param[in] pCtx             The decode context.
  @param[in] uTagRequirement  One of @c QCBOR_TAG_REQUIREMENT_XXX.
- @param[out] pDateString            The decoded URI.
+ @param[out] pDateString     The decoded date.
 
  This decodes the standard CBOR date/time string tag, integer tag
  number of 0, or encoded CBOR that is not a tag, but borrows the
@@ -1020,6 +1020,40 @@
 
 
 /**
+ @brief Decode the next item as a date-only string.
+
+ @param[in] pCtx             The decode context.
+ @param[in] uTagRequirement  One of @c QCBOR_TAG_REQUIREMENT_XXX.
+ @param[out] pDateString     The decoded date.
+
+ This decodes the CBOR date-only string tag, integer tag
+ number of 1004, or encoded CBOR that is not a tag, but borrows the
+ date-only string content format. An example of the format
+ is "1985-04-12".
+
+ See @ref Decode-Errors for discussion on how error handling works.
+
+ See @ref Tag-Usage for discussion on tag requirements.
+
+ See also @ref CBOR_TAG_DAYS_STRING, QCBOREncode_AddDaysString() and
+ @ref QCBOR_TYPE_DAYS_STRING.
+*/
+static void QCBORDecode_GetDaysString(QCBORDecodeContext *pCtx,
+                                      uint8_t             uTagRequirement,
+                                      UsefulBufC         *pDateString);
+
+static void QCBORDecode_GetDaysStringInMapN(QCBORDecodeContext *pCtx,
+                                            int64_t             nLabel,
+                                            uint8_t             uTagRequirement,
+                                            UsefulBufC         *pDateString);
+
+static void QCBORDecode_GetDaysStringInMapSZ(QCBORDecodeContext *pCtx,
+                                             const char         *szLabel,
+                                             uint8_t             uTagRequirement,
+                                             UsefulBufC         *pDateString);
+
+
+/**
  @brief Decode the next item as an epoch date.
 
  @param[in] pCtx             The decode context.
@@ -1063,6 +1097,40 @@
                                      int64_t            *pnTime);
 
 
+/**
+ @brief Decode the next item as an days-count epoch date.
+
+ @param[in] pCtx             The decode context.
+ @param[in] uTagRequirement  One of @c QCBOR_TAG_REQUIREMENT_XXX.
+ @param[out] pnDays          The decoded epoch date.
+
+ This decodes the CBOR epoch date tag, integer tag number of 100, or
+ encoded CBOR that is not a tag, but borrows the content format. The
+ date is the number of days (not number of seconds) before or after
+ Jan 1, 1970.
+
+ See @ref Decode-Errors for discussion on how error handling works.
+
+ See @ref Tag-Usage for discussion on tag requirements.
+
+ See also @ref CBOR_TAG_DAYS_EPOCH, QCBOREncode_AddTDaysEpoch() and
+ @ref QCBOR_TYPE_DAYS_EPOCH.
+*/
+void QCBORDecode_GetEpochDays(QCBORDecodeContext *pCtx,
+                              uint8_t             uTagRequirement,
+                              int64_t            *pnDays);
+
+void QCBORDecode_GetEpochDaysInMapN(QCBORDecodeContext *pCtx,
+                                    int64_t             nLabel,
+                                    uint8_t             uTagRequirement,
+                                    int64_t            *pnDays);
+
+void QCBORDecode_GetEpochDaysInMapSZ(QCBORDecodeContext *pCtx,
+                                     const char         *szLabel,
+                                     uint8_t             uTagRequirement,
+                                     int64_t            *pnDays);
+
+
 
 
 /**
@@ -2171,6 +2239,54 @@
    QCBORDecode_GetTaggedStringInMapSZ(pMe, szLabel, TagSpec, pText);
 }
 
+static inline void
+QCBORDecode_GetDaysString(QCBORDecodeContext *pMe,
+                          uint8_t             uTagRequirement,
+                          UsefulBufC         *pValue)
+{
+   const TagSpecification TagSpec =
+      {
+         uTagRequirement,
+         {QCBOR_TYPE_DAYS_STRING, QCBOR_TYPE_NONE, QCBOR_TYPE_NONE, QCBOR_TYPE_NONE},
+         {QCBOR_TYPE_TEXT_STRING, QCBOR_TYPE_NONE, QCBOR_TYPE_NONE, QCBOR_TYPE_NONE}
+      };
+
+   QCBORDecode_GetTaggedStringInternal(pMe, TagSpec, pValue);
+}
+
+inline static void
+QCBORDecode_GetDaysStringInMapN(QCBORDecodeContext *pMe,
+                                int64_t             nLabel,
+                                uint8_t             uTagRequirement,
+                                UsefulBufC         *pText)
+{
+   const TagSpecification TagSpec =
+      {
+         uTagRequirement,
+         {QCBOR_TYPE_DAYS_STRING, QCBOR_TYPE_NONE, QCBOR_TYPE_NONE, QCBOR_TYPE_NONE},
+         {QCBOR_TYPE_TEXT_STRING, QCBOR_TYPE_NONE, QCBOR_TYPE_NONE, QCBOR_TYPE_NONE}
+      };
+
+   QCBORDecode_GetTaggedStringInMapN(pMe, nLabel, TagSpec, pText);
+}
+
+inline static void
+QCBORDecode_GetDaysStringInMapSZ(QCBORDecodeContext *pMe,
+                                 const char         *szLabel,
+                                 uint8_t             uTagRequirement,
+                                 UsefulBufC         *pText)
+{
+   const TagSpecification TagSpec =
+      {
+         uTagRequirement,
+         {QCBOR_TYPE_DAYS_STRING, QCBOR_TYPE_NONE, QCBOR_TYPE_NONE, QCBOR_TYPE_NONE},
+         {QCBOR_TYPE_TEXT_STRING, QCBOR_TYPE_NONE, QCBOR_TYPE_NONE, QCBOR_TYPE_NONE}
+      };
+
+   QCBORDecode_GetTaggedStringInMapSZ(pMe, szLabel, TagSpec, pText);
+}
+
+
 
 static inline void QCBORDecode_GetURI(QCBORDecodeContext *pMe,
                                       uint8_t             uTagRequirement,