Question

I am not interested in what the current UTC time is in milliseconds, nor do I need to mess with timezones. My original date is already stored as a UTC timestamp.

I have a date stored in a database in UTC time, “2012-06-14 05:01:25”.
I am not interested in the datetime, but just the date portion of the it. So, after retrieving the date in Java, and excluding the hours, minutes, and seconds – I am left with “2012-06-14”.

How can I convert this into UTC milliseconds?

Answer 1

EDIT: I’d missed the “ignoring the time of day” part. It’s now present, but near the end…

The simplest approach is probably to use SimpleDateFormat, having set the time zone appropriately:

SimpleDateFormat format = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss", Locale.US);
format.setTimeZone(TimeZone.getTimeZone("UTC"));

Date date = format.parse(text);
long millis = date.getTime();

(Setting the time zone is the important bit here, as otherwise it will interpret the value to be in the local time zone.)

Alternatively, if you’re doing anything less trivial than this, use Joda Time which is a much better date/time API. In particular, SimpleDateFormat isn’t thread-safe whereas DateTimeFormatter is:

// This can be reused freely across threads after construction.
DateTimeFormatter formatter = DateTimeFormat.forPattern("yyyy-MM-dd HH:mm:ss")
    .withLocale(Locale.US)
    .withZoneUTC();

// Option 1
DateTime datetime = formatter.parseDateTime(text);
long millis = dateTime.getMillis();

// Option 2, more direct, but harder to diagnose errors
long millis = formatter.parseMillis(text);

Now so far, we’ve parsed the whole whole caboodle. The easiest way of ignoring the date part is just to round it off – after all, Java doesn’t observe leap seconds, so we can just truncate it:

long millisPerDay = 24L * 60L * 60L * 1000L; // Or use TimeUnit
long dayMillis = (millis / millisPerDay) * millisPerDay;

That will “round towards 1970” so if you have a date before 1970 it will round to the end of the day – but I suspect that’s unlikely to be a problem.

With the Joda Time version you could just use this instead:

DateTime dateTime = formatter.parseDateTime(text);
long millis = dateTime.toLocalDate().getLocalMillis();

I would personally not go with the idea of just taking a substring. Even though you’re not actually interested in preserving the hour/minute/second, I think it’s appropriate to parse what you’ve been given and then throw away information. Aside from anything else, it makes your code fail appropriately with bad data, e.g.

"2012-06-100"

or

"2012-06-14 25:01:25"

indicate problems in whatever’s supplying you data, and it’s good to spot that rather than to continue blindly just because the first 10 characters are okay.