Here's the updated code with a unixtime stamp...
Serial output:
dht with sd rtc logging
Created: 14:17:20, Jul 26 2019
C:\Users\xxx\Documents\Arduino\dht11_lcd_v5_minmax_pulse_rtc_sd\dht11_lcd_v5_minmax_pulse_rtc_sd.ino
Initializing SD card...card initialized.
setup() complete
Writing to SD: 1564150659,16,37
Writing to SD: 1564150662,17,37
Writing to SD: 1564150667,17,36
SD card on PC:
1564150653,16,38
1564150659,16,37
1564150662,17,37
1564150667,17,36
SD card opened in Excel and with Excel converting unixtime to humantime (here's how):
1564150653 14:17:33 26-07-19 16 38
1564150659 14:17:39 26-07-19 16 37
1564150662 14:17:42 26-07-19 17 37
1564150667 14:17:47 26-07-19 17 36
Updated sketch:
// dht11 with lcd
// v1 26 march 2017
//
//this version diplays min and max T since power up
//lcd backlight is operated by a button
//with pulse, and no delay on the dht read
//v5 with rtc and sd log inspired by https://forum.arduino.cc/index.php?topic=627958
// todo... rtc, so far only sd log of dht... DONE
//26 july 2019
//dht stuff
#include <dht11.h>
dht11 DHT;
#define DHT11_PIN 2
//wire and lcd and rtc stuff
#include <Wire.h>
#include <LiquidCrystal_I2C.h>
LiquidCrystal_I2C lcd(0x27, 16, 2);
#include <DS3231.h>
RTClib RTC;
DateTime now;
//sd card stuff
#include <SPI.h>
#include <SD.h>
const int chipSelect = 4;
String dataString;
byte minTemp = 99;
byte maxTemp = 0;
byte currTemp;
byte lightPin = 6;
//pulse stuff
int heartbeatInterval = 1000;
bool showHeart = true;
unsigned long previousMillis_heartbeat = 0;
//delayless dht read
int dhtInterval = 5000;
unsigned long previousMillis_dht = 0;
//delayless sd write
int sdInterval = 5000;
unsigned long previousMillis_sd = 0;
void setup()
{
 //print some file details
 Serial.begin(9600);
 Serial.println("dht with sd rtc logging");
 Serial.print("Created: ");
 Serial.print(__TIME__);
 Serial.print(", ");
 Serial.println(__DATE__);
 Serial.println(__FILE__);
 //turn off l13
 pinMode(13, OUTPUT);
 digitalWrite(13, LOW);
 //splash screen
 pinMode(lightPin, INPUT_PULLUP);
 lcd.begin();
 lcd.backlight();
 lcd.print("  DHT11");
 lcd.setCursor(0, 1);//along, down
 lcd.print(" station v5");
 delay(1000);
 //lcd.noBacklight();
 //lcd.clear();
 //initialise sd card
 Serial.print("Initializing SD card...");
 // see if the card is present and can be initialized:
 if (!SD.begin(chipSelect))
 {
  Serial.println("Card failed, or not present");
  lcd.setCursor(0, 1);//along, down
  lcd.print(" ** SD fail **");
  // don't do anything more:
  while (1);
 }
 Serial.println("card initialized.");
 lcd.clear();
 //need to read once to get some proper data before the delayless reads
 DHT.read(DHT11_PIN);  // READ DATA
 now = RTC.now();
 Serial.println("setup() complete"); Serial.println(" ");
}
void loop()
{
 if (millis() - previousMillis_heartbeat > heartbeatInterval)
 {
  doPulse();
 }
 //read dht without delay
 if (millis() - previousMillis_dht > dhtInterval)
 {
  DHT.read(DHT11_PIN);  // READ DATA
  now = RTC.now();
  previousMillis_dht = millis();
 }
 //display dht on lcd
 doLCD();
 //update sd without delay
 if (millis() - previousMillis_sd > sdInterval)
 {
  //fakeUpdateSD();
  createStringForSD();
  writeStringToSD();
  previousMillis_sd = millis();
 }
 if (!digitalRead(lightPin))
 {
  lcd.backlight();
 }
 else
 {
  lcd.noBacklight();
 }
}//loop
void writeStringToSD()
{
 // open the file. note that only one file can be open at a time,
 // so you have to close this one before opening another.
 File dataFile = SD.open("databla3.txt", FILE_WRITE);
 // if the file is available, write to it:
 if (dataFile)
 {
  dataFile.println(dataString);
  dataFile.close();
  // print to the serial port too:
  Serial.print("Writing to SD: ");
  Serial.println(dataString);
 }
 // if the file isn't open, pop up an error:
 else {
  Serial.println("error opening databla3.txt");
 }
}
void createStringForSD()
{
 // make a string for assembling the data to log:
 dataString = "";
 //get unixtime and append to string
 //  could use now.year() etc etc
 dataString += String(now.unixtime());
 dataString += ",";
 // get T & H and append to string:
 dataString += String(DHT.temperature);
 dataString += ",";
 dataString += String(DHT.humidity);
 //Serial.println(dataString);
}//createStringForSD
void fakeUpdateSD()
{
 Serial.println("fake write to SD to test timing");
}
void doLCD()
{
 currTemp = DHT.temperature;
 lcd.setCursor(0, 0); //along, down
 lcd.print("Temp:");
 if (currTemp < 10) lcd.print(" ");
 lcd.print(currTemp);
 lcd.print((char)223); //degree symbol
 lcd.print("C RH:");
 lcd.print(DHT.humidity);
 lcd.print("%");
 if (currTemp < minTemp) minTemp = currTemp;
 if (currTemp > maxTemp) maxTemp = currTemp;
 lcd.setCursor(0, 1);//along, down
 lcd.print("Min:");
 if (minTemp < 10) lcd.print(" ");
 lcd.print(minTemp);
 lcd.print((char)223);
 lcd.setCursor(9, 1);//along, down //jump over pulse indicator
 lcd.print("Max:");
 if (maxTemp < 10) lcd.print(" ");
 lcd.print(maxTemp);
 lcd.print((char)223);
}//doLCD
void doPulse()
{
 showHeart = !showHeart;
 previousMillis_heartbeat = millis();
 lcd.setCursor(7, 1);
 if (showHeart)
 {
  lcd.print(". ");
 }
 else
 {
  lcd.print(" .");
 }
} //doPulse
