Add date time offset (set time Zone)

Hi

Iam using CUBEMX and kiel for my development. iam read date time from the GSM modem

when i read it from the modem its looks like this.

+QLTS: "20/09/24,18:08:18+22,0"

+22 mean my time zone(+5:30). my actual time at this time is

2020:09:24 23:38:20

i will extract year, month..... from the above string and pass it into another function

void update_Internal_RTC(Time_t set_time){
	
 	
 RTC_TimeTypeDef set_Time = {0};
 RTC_DateTypeDef set_Date = {0};
		
 set_Time.Hours = set_time.Hour;
 set_Time.Minutes = set_time.Minute;
 set_Time.Seconds = set_time.Second;
 
 if (HAL_RTC_SetTime(&hrtc, &set_Time, RTC_FORMAT_BIN) != HAL_OK)
 {
 Error_Handler();
 }
	
 set_Date.Month = time.Month;
 set_Date.Date = time.Date;
 set_Date.Year = time.Year;
 printf("year is %d\r\n",time.Year);
 if (HAL_RTC_SetDate(&hrtc, &set_Date, RTC_FORMAT_BIN) != HAL_OK)
 {
 Error_Handler();
 }
	
	 	
}

now i want to add my time offset before sent it to RTC module. simply i need add 19800 (5:30) seconds before save it on the timer. what is the simplest way

Greetings