help using the time library on ESP32

[johnerrington]

I'm very confused as to how to get values from the time library.

The SimpleTime example is exactly NO help.

Firstly, there are many time.h files on my system;
there seem to be seven different versions; 18643; 7203; 2699; 2676; 1828; 399; and 21 bytes

The general idea is that they SEEM to create a structure holding these values

struct tm {
int8_t tm_sec; /< seconds after the minute - [ 0 to 59 ] */
int8_t tm_min; /< minutes after the hour - [ 0 to 59 ] */
int8_t tm_hour; /< hours since midnight - [ 0 to 23 ] */
int8_t tm_mday; /< day of the month - [ 1 to 31 ] */
int8_t tm_wday; /< days since Sunday - [ 0 to 6 ] */
int8_t tm_mon; /< months since January - [ 0 to 11 ] */
int16_t tm_year; /< years since 1900 */
int16_t tm_yday; /< days since January 1 - [ 0 to 365 ] */
int16_t tm_isdst; /**< Daylight Saving Time flag */
};
https://www.cplusplus.com/reference/ctime/tm/

From the SimpleTime example I can print the date and time like this

Serial.println(&timeinfo, "%A, %B %d %Y %H:%M:%S");

but I'm totally at a loss HOW that works. Although I have found the "format specifiers"
http://www.cplusplus.com/reference/ctime/strftime/

What I'm TRYING to do is

1: to create a string (yyyy_mm.csv eg 2021_04.csv) that I can use as a filename in SPIFFS;
I've now discovered I can do this:

 strftime (buffer, 80, "%Y_%m.csv", &timeinfo); //4 digit year, 2 digit month
  Serial.print("Filename is ");
  Serial.println(buffer);

which outputs Filename is 2021_04.csv

2: to get the values of day, hour, minute, second as integers.

Any help greatly appreciated!

[guix]

For that Serial.print, look at Print.cpp

In the SimpleTime example, there is this:

  struct tm timeinfo;
  if(!getLocalTime(&timeinfo)){
    Serial.println("Failed to obtain time");
    return;
  }
  Serial.println(&timeinfo, "%A, %B %d %Y %H:%M:%S");

timeinfo is an empty tm object, then its values are set by calling getLocalTime

You can then use timeinfo.tm_hour etc

[johnerrington]

Thanks @guix that all works. it would be great if someone would write a tutorial on using the time functions though as there is a LOT of conflicting info on the web. (I must have seen most of it!)

 char buffer [80];
  strftime (buffer, 80, "%Y_%m.csv", &timeinfo); //4 digit year, 2 digit month
  Serial.print("Filename is ");
  Serial.println(buffer);
  int my_seconds = timeinfo.tm_sec;  //gets seconds as an integer
  Serial.println(my_seconds);

thanks for the link - this explains a LOT (that the arduino Serial.print doesnt!)

size_t Print::print(struct tm * timeinfo, const char * format)
{
    const char * f = format;
    if(!f){
        f = "%c";
    }
    char buf[64];
    size_t written = strftime(buf, 64, f, timeinfo);
    if(written == 0){
        return written;
    }
    return print(buf);
}

?Is there an easy way to check if the latest sychronization to NTP was successful?