AP_RTC: add local time support

libraries/AP_RTC/AP_RTC.cpp

@@ -28,6 +28,13 @@ const AP_Param::GroupInfo AP_RTC::var_info[] = {
     // @User: Advanced
     AP_GROUPINFO("_TYPES",  1, AP_RTC, allowed_types, 1),
 
+    // @Param: _TZ_MIN
+    // @DisplayName: Timezone offset from UTC
+    // @Description: Adds offset in +- minutes from UTC to calculate local time
+    // @Range: -720 +840
+    // @User: Advanced
+    AP_GROUPINFO("_TZ_MIN",  2, AP_RTC, tz_min, 0),
+    
     AP_GROUPEND
 };
 
@@ -102,6 +109,31 @@ bool AP_RTC::get_system_clock_utc(uint8_t &hour, uint8_t &min, uint8_t &sec, uin
     return true;
 }
 
+bool AP_RTC::get_local_time(uint8_t &hour, uint8_t &min, uint8_t &sec, uint16_t &ms)
+{
+     // get local time of day in ms
+    uint64_t time_ms = 0;
+    uint64_t ms_local = 0;
+    if (!get_utc_usec(time_ms)) {
+        return false;
+    }
+    time_ms /= 1000U;
+    ms_local = time_ms + (tz_min * 60000);
+
+    // separate time into ms, sec, min, hour and days but all expressed in milliseconds
+    ms = ms_local % 1000;
+    uint32_t sec_ms = (ms_local % (60 * 1000)) - ms;
+    uint32_t min_ms = (ms_local % (60 * 60 * 1000)) - sec_ms - ms;
+    uint32_t hour_ms = (ms_local % (24 * 60 * 60 * 1000)) - min_ms - sec_ms - ms;
+
+    // convert times as milliseconds into appropriate units
+    sec = sec_ms / 1000;
+    min = min_ms / (60 * 1000);
+    hour = hour_ms / (60 * 60 * 1000);
+
+    return true;
+}
+
 // get milliseconds from now to a target time of day expressed as
 // hour, min, sec, ms.  Match starts from first value that is not
 // -1. I.e. specifying hour=-1, minutes=10 will ignore the hour and

libraries/AP_RTC/AP_RTC.h

@@ -13,6 +13,7 @@ public:
     static const struct AP_Param::GroupInfo var_info[];
 
     AP_Int8 allowed_types;
+    AP_Int16 tz_min;
 
     // ordering is important in source_type; lower-numbered is
     // considered a better time source.  These values are documented
@@ -38,6 +39,8 @@ public:
       get time in UTC hours, minutes, seconds and milliseconds
      */
     bool get_system_clock_utc(uint8_t &hour, uint8_t &min, uint8_t &sec, uint16_t &ms);
+    
+    bool get_local_time(uint8_t &hour, uint8_t &min, uint8_t &sec, uint16_t &ms);
 
     uint32_t get_time_utc(int32_t hour, int32_t min, int32_t sec, int32_t ms);