How to store datetime in sketch

Projects General Guidance
1

Until now my sketch has stored datetime for display on a screen in a character array like so;

char serverdatetime[16];

27 Jan 7:40 pm

Yes I know it's a horrible waste of SRAM!

Apart from wanting to store datetime properly, my sketch will eventually need to know what day of the week it is.

I don't need the sketch to keep time itself, and I'm not using a RTC. Instead the Arduino is receiving a datetime message from the server every 30 seconds, in exactly the format above, so I'm just copying the string into a char array right now. (Obviously millisecond accuracy is not critical to this application.)

I'm aware of the Arduino DateTime library, but given I don't need the sketch to keep its own time, would it be more memory efficient to store the date and time in my own struct, then make some basic calculations to work out what day of the week it is (i.e. 1=Monday)?

Or should I just use the DateTime library and update the value every time the MQTT message is received?

2

Days of the month could be BCD, as could hours and minutes.
Month could be a single hex digit.
am/pm is a single bit.
Year could be a simple offset from an arbitrary year zero.

Or just store the whole lot in a single unsigned long as the number of seconds since Jan 1st 1970.

3

hazymat:
Apart from wanting to store datetime properly, my sketch will eventually need to know what day of the week it is.

The day of the week on 27th of January depends on the year.

If the sketch knows the year, it's easy to calculate the day of the week.

But day of the week on 27th of January in 2016 is not the same as 2015 or 2017.

4

I'm leaning towards AWOL's suggestion of storing in BCD. Then writing an Arduino function to calculate the day of the week based on the Doomsday algorithm. It's a silly amount of work, but fun...