Question

I have some things in my Android application that need to update once per day.

It’s pretty simple I think, but I have no idea in what format I need to format the date and time (if time is needed) and how to check if an update has been done today (where today is between 00:01am and 23:59pm in the user’s local time). The update should not be done if it was already done for today.

Here’s what I DO know how to do:

• Save the last update date in SharedPreferences (but how do I get a
  string of it, I do not know)
• Get things from SharedPreferences (but I
  don’t know how to compare dates in string format)

Answer 1

It is irrelevant what format you choose. It is just matter of recalculations.

I’d suggest using milliseconds since epoch, as all system calls use it, so it would be easier for you to use the same.

As 1000 millis is 1 second it’s easy to figure out that 1000*60*60*24 equals to one day (24hrs). So, if storedMillis is bigger than NOW - 1000*60*60*24, (and NOW is i.e. System.currentTimeMillis()), then it is too early to do the check. If storedMillis is smaller, then save your NOW timestamp and do the check:

long now = System.currentTimeMillis();
long diffMillis = now - lastCheckedMillis;
if( diffMillis >= (3600000  * 24) ) {
  // store now (i.e. in shared prefs)

  // do the check
} else {
   // too early
}

EDIT

I am interested in doing it when the app is first opened for the
current day, even if the last update was done 10 minutes ago.

It’s just the matter how to get the proper millis to compare against. Replace long now = System.currentTimeMillis(); from above code with following code block:

Calendar cal = Calendar.getInstance();
cal.clear(Calendar.HOUR);
cal.clear(Calendar.HOUR_OF_DAY);
cal.clear(Calendar.MINUTE);
cal.clear(Calendar.SECOND);
cal.clear(Calendar.MILLISECOND);

long now = cal.getTimeInMillis();

which shall do the trick.