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I'm storing dates and times as one of the fields in my sql database in the following format: dd/mm/yy hh:mm. I am importing this info into my android app with an asynctask and then populating a listview.

What I need to do is to check whether or not the date imported is after or before the current date and time. All the data in my database is in GMT and I am wondering what method should I call in java to get the best representation of the current date and time independent of the timezone the user is in, so that I can carry out the comparison.

My main concern is to avoid the user being able to cheat the app by changing the system time on his phone.

Basil Bourque
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Alk
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2 Answers2

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Get Current Date-Time Elsewhere

If you cannot trust the current date-time on the user’s device/computer, then you have two alternatives:

  • Get the current date-time from somewhere else.
    • See this Question or this Question for getting current date-time from time servers using Java/Android.
    • See this Question for getting current date-time from your database server.
  • Let your database server use its own date-time when searching for records.
    See Questions like this or this. Pay careful attention to the various date-time functions offered by your particular database. For example, some commands return the moment when the transactions began while others return the current moment of execution. A few commands are standard SQL while most are proprietary. As an example, see the Postgres date-time functions doc.

Joda-Time

If you get the time from elsewhere, then you need to work with the date-time values in Java. When doing such work, use a good date-time library. For Android that means the Joda-Time library. Avoid the bundled java.util.Date/.Calendar as they are notoriously troublesome.

There are many other Questions and Answers on StackOverflow on this topic, but here's a quick untested example. I assume you are using a JDBC driver to obtain a java.sql.Timestamp value from your database.

java.sql.Timestamp ts = myResultSet.getTimestamp( "my_column_" );  // Get date-time object from your database via JDBC driver.
…
DateTime now = … // Get current date-time from other computer as discussed above.
DateTime when = new DateTime( ts , DateTimeZone.UTC );  // Convert java.sql.Timestamp to org.joda.time.DateTime object.
Boolean rowIsPast = when.isBefore( now );  // Compare DateTime objects.

If you are getting string representations of your data’s date-time values from the database, try to instead get an object via JDBC, a java.sql.Timestamp (or in the future, an object of a type defined by the new java.time package in Java 8 and later). If you must work with string representations, search for many examples on StackOverflow.com of parsing strings into date-time values in Java & Joda-Time (use the search term "joda").

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Basil Bourque
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String time1 = "16:00:00";
String time2 = "19:00:00";

SimpleDateFormat format = new SimpleDateFormat("HH:mm:ss");
Date date1 = format.parse(time1);
Date date2 = format.parse(time2);
long difference = date2.getTime() - date1.getTime();

This would give you the time difference in seconds. Check to see if that is positive to see which date is "first"

JShell
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  • Would this also take into account the date as well as ignore the system time of the user? My main concern is to avoid the user being able to cheat the app by changing the system time on his phone. – Alk Jun 10 '15 at 17:54
  • This doesn't take into account that one of the dates is in GMT. – shmosel Jun 10 '15 at 18:16
  • @mankee You'd have to modify the strings slightly to include dates. Whether or not this can be abused depends entirely on how your code retrieves those dates, not the conversion. – JShell Jun 10 '15 at 18:20
  • @JesseShellabarger I launch an AsyncTask in the oncreate method which retrieves the date from my online database, next step is do the comparison, if the date is after the current date do not display the item in the listview, if it is before the current date display the item – Alk Jun 10 '15 at 18:24
  • @makee I guess it depends on how phones get their times then. I'm not familiar with how this works, but i believe it pulls this data from the carrier, yes? – JShell Jun 10 '15 at 18:40
  • @Jesse Shellabarger They have that as an option, but they also have set manually as well... the best thing would be if I could get the current date from an external source like a server – Alk Jun 10 '15 at 18:47