Why Does SimpleDateFormat.parse Produce Unexpected Output When Converting UNIX Timestamps?

Different Output Formats from SimpleDateFormatter.parse

In a Java program, the SimpleDateFormat class is utilized to convert dates into specified formats. However, it has been observed that in some cases, parsing a date using SimpleDateFormatter.parse can result in a different format than the one specified during formatting.

This issue arises when converting UNIX timestamps into date strings. Here's a simplified example:

<code class="java">String ep ="a1527069600";
Long epoch = Long.parseLong(ep.substring(1, ep.length()));
Date dt = new Date(epoch*1000L);
SimpleDateFormat sdf = new SimpleDateFormat("dd/MM/yyyy hh:mm:ss a");
sdf.setTimeZone(TimeZone.getTimeZone("Asia/Calcutta"));

System.out.println("SDF.Format - "+sdf.format(dt));
System.out.println("SDF.parse - "+sdf.parse(sdf.format(dt)));</code>

In this example, while the format specified for sdf is "dd/MM/yyyy hh:mm:ss a", the output of sdf.parse is in a different format, "Wed May 23 15:30:00 IST 2018."

Solution

To resolve this issue, avoid passing date strings to a MySQL database. Instead, use date objects. Java provides the java.time API for modern date and time handling. For your specific scenario, you can use a LocalDateTime object:

<code class="java">    String ep ="a1527069600";
    long epoch = Long.parseLong(ep.substring(1));
    Instant inst = Instant.ofEpochSecond(epoch);
    LocalDateTime ldt = inst.atZone(ZoneId.of("Asia/Calcutta")).toLocalDateTime();

    PreparedStatement ps = myDatabaseConnection.prepareStatement(
            "insert into my_table (my_date_time) values (?)");
    ps.setObject(1, ldt);</code>

This approach ensures consistent date formatting and eliminates the parsing issue encountered with SimpleDateFormatter.parse.

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