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I'm trying to use an if statement to check when the time goes past a specific point and run a specific function. To do this I'm looking to convert time into a double and running something along the lines of if(timeCurrent >= 8.30). I seem to however have an issue with the conversion part, normally I'm aware the standard practice in C++ would be to do the following: 

DateTime now = rtc.now();
static char buffer[80];

sprintf(buffer, "%02d.%02d", now.hour(), now.minute());
String str(buffer);
double timeCurrent = strtod(str);

However, this doesn't work for Arduino. What can be done to create the same desired function?

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  • this may help ... you made no mention of any research that you may have done ... playground.arduino.cc/Code/Time
    – jsotola
    May 24, 2018 at 22:35
  • convert buffer to float, not String. but there are more elegant end efficient solutions. if you really want float then 0.01 * m + h
    – Juraj ♦
    May 25, 2018 at 3:51
  • There is really no need to take the extra effort in converting first to char buffer, then String and then double. Just convert the ints from the RTC directly to a float like juraj wrote. And I doubt, that your code is a good practice in C++ either
    – chrisl
    May 25, 2018 at 7:26
  • I'm aware the standard practice in C++ would be to do the following... Umm... no, it certainly would not. What would give you that impression?
    – Majenko ♦
    May 25, 2018 at 9:35

1 Answer 1

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Don't do that. Converting a time to double is highly inefficient. Moreover, your approach suffers from a rollover issue every 24h.

If you just want to compare the time of day with 8:30, you can do it in a very straightforward way:

if (now.hour() > 8 || (now.hour() == 8 && now.minute() >= 30))

If the long if annoys you, you can put the comparison logic inside a static inline function. If you really want to combine hour and minute into a single number, then convert to an integer rather than a double:

if (now.hour() * 100 + now.minute() >= 830)

This can be made more efficient if you think in binary rather than decimal: you can build a 16-bit number with one byte holding the hours and the other byte holding the minutes:

if ((now.hour() << 8) | now.minute() >= (8 << 8) | 30)

Again, you may encapsulate the ((a)<<8|(b)) in a macro or static inline function if it helps with readability. If you want to compare to an absolute time, your best option is to build a DateTime object representing the deadline, and compare with now as:

// Assume timeout should be today at 8:30.
DateTime timeout(now.year(), now.month(), now.day(), 8, 30, 0);
if (now.secondstime() >= timeout.secondstime())

Here the secondstime() method is used to convert the DateTime into a scalar number (a 32-bit integer of type time_t) that can be easily compared and is far more efficient than using floats.

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