Minimizing Memory Consumption on sscanf - c

Question

I have a time&date string acquired from a GSM module.
Here is the time&date format:
"yy/MM/dd,hh:mm:ss±zz"
And an example string:
"+CCLK: "17/04/07,12:57:43+03""
I need to parse it and convert it to Unix Timestamp. I have the below implementation for it and it works well:
Time Struct:
struct timeStruct{
uint16 Year;
uint8 Month;
uint8 Day;
uint8 Hour;
uint8 Minute;
uint8 Second;
};
GSM Clock Command Parser:
void load_clock(void)
{
struct timeStruct gsmTimeStruct;
/* Parse year,month,day,hour,minute,second and GMT timezone */
sscanf(gsm_clk_string, "+CCLK: \"%hd/%d/%d,%d:%d:%d%c%d", &(gsmTimeStruct.Year), &(gsmTimeStruct.Month), &(gsmTimeStruct.Day), &(gsmTimeStruct.Hour), &(gsmTimeStruct.Minute), &(gsmTimeStruct.Second),&CONEZONE,&TIMEZONE);
unixTime = (uint32_t)RTC_DateTimeToUnix(gsmTimeStruct);
/* Collect GMT difference */
if(CONEZONE == '+')
unixTime = unixTime - TIMEZONE*3600;
else
unixTime = unixTime + TIMEZONE*3600;
}
That sscanf function consumes 1776 Bytes from the Flash Memory which is a huge number for this MCU. The Unix conversion function also consumes about 700 Bytes but it is a bit normal as it has some mathematical calculations in it.
Are there any bright ideas on parsing this string without consuming such big flash memory usage?
Note: The error checking for the Command String is not an issue since the GSM module already does it. So, if we want to change the parsing implementation, we don't have to do it. We can just assume those numbers are always there.
Edit: I tried below snippet, it works well. It saved 1.7 kilobytes memory but I'm not sure if this is a proper way.
gsmTimeStruct.Year = (gsm_clk_string[8]-'0')*10 + (gsm_clk_string[9]-'0');
gsmTimeStruct.Month = (gsm_clk_string[11]-'0')*10 + (gsm_clk_string[12]-'0');
gsmTimeStruct.Day = (gsm_clk_string[14]-'0')*10 + (gsm_clk_string[15]-'0');
gsmTimeStruct.Hour = (gsm_clk_string[17]-'0')*10 + (gsm_clk_string[18]-'0');
gsmTimeStruct.Minute = (gsm_clk_string[20]-'0')*10 + (gsm_clk_string[21]-'0');
gsmTimeStruct.Second = (gsm_clk_string[23]-'0')*10 + (gsm_clk_string[24]-'0');
TIMEZONE = (gsm_clk_string[26]-'0')*10 + (gsm_clk_string[27]-'0');
CONEZONE = gsm_clk_string[25];

Answer 1

I would think all you'd need is this, just move the char ptr accordingly.
In code example below i assume the first character in gsm_clk_string is "
/* "+CCLK: "2017/04/07, 12:57:43+03"" */
/* gsm_clk_string[] : 0123456789012345678902134567890123 */
char *ptr;
ptr = gsm_clk_string;
ptr += 9;
gsmTimeStruct.Year = (uint16) ((*ptr - '0') * 1000);
ptr++;
gsmTimeStruct.Year += (uint16) ((*ptr - '0') * 100);
ptr++;
gsmTimeStruct.Year += (uint16) ((*ptr - '0') * 10);
ptr++;
gsmTimeStruct.Year += (uint16) ((*ptr - '0'));
ptr += 2;
gsmTimeStruct.Month = (uint8) ((*ptr - '0') * 10);
ptr++;
gsmTimeStruct.Month += (uint8) (*ptr - '0');
ptr += 2;
gsmTimeStruct.Day = (uint8) ((*ptr - '0') * 10);
ptr++;
gsmTimeStruct.Day += (uint8) (*ptr - '0');
ptr += 3;
gsmTimeStruct.Hour = (uint8) ((*ptr - '0') * 10);
ptr++;
gsmTimeStruct.Hour += (uint8) (*ptr - '0');
ptr += 2;
gsmTimeStruct.Minute = (uint8) ((*ptr - '0') * 10);
ptr++;
gsmTimeStruct.Minute += (uint8) (*ptr - '0');
ptr += 2;
gsmTimeStruct.Second = (uint8) ((*ptr - '0') * 10);
ptr++;
gsmTimeStruct.Second += (uint8) (*ptr - '0');
ptr++;
if ( *ptr == '+' )
{
ptr++;
TIMEZONE = (whatever_type_TimeZone) ((*ptr - '0') * 10);
ptr++
TIMEZONE += (whatever_type_TimeZone) ((*ptr - '0'));
unixTime = (uint32_t) RTC_DateTimeToUnix( gsmTimeStruct ) - ( TIMEZONE * 3600 );
}
else if ( *ptr == '-' )
{
ptr++;
TIMEZONE = (whatever_type_TimeZone) ((*ptr - '0') * 10);
ptr++
TIMEZONE += (whatever_type_TimeZone) ((*ptr - '0'));
unixTime = (uint32_t) RTC_DateTimeToUnix( gsmTimeStruct ) + ( TIMEZONE * 3600 );
}
else
{
/* problem */
}