Heap Memory usage growth cause overflow

I try reading meter time (clock), and the value is read successfully, then print it to serial console, I use:
//Read clock.
ret = com_read(&clock1.base, 2);
if (ret != DLMS_ERROR_CODE_OK)
{
return;
}

Serial.printf("%02d-%02d-%04d %02d:%02d:%02d\r\n",
clock1.time.value.tm_mday,
clock1.time.value.tm_mon,
clock1.time.value.tm_year + 1900,
clock1.time.value.tm_hour,
clock1.time.value.tm_min,
clock1.time.value.tm_sec);

and the output is correct: "14-09-2019 16:15:50"

were I must put "var_clear()", and what value must be passed to it?
is there any example?

Regards,
Fery_rz

Thank you Mikko,
I use this, the OBIS i get from GXDLMSDirector.
gxClock clock1;
cosem_init(&clock1.base, DLMS_OBJECT_TYPE_CLOCK, "0.0.1.0.0.255");

when I try to monitor the heap, befor and after calling "ret = com_read(&clock1.base, 2);", then subtract it, it use 144 bytes.
I try reading another value from the meter, for example I read the meter serial number using "com_read(&data1.base, 2);" it used different size.

and the free heap always decreased after we call it function, that use "com_read" inside,.
how about "var_clear()", is there any example usage of it?

Best regards,
Fery