Get starting date of week(configurable starting day of week)

Question

I have current date, and a constant which tells from which day the week starts. I want to get the start date of the week based on that constant. If I hardcode the first day

Answer 1

tl;dr

LocalDate.now( ZoneId.of( "America/Montreal" ) )
         .with( TemporalAdjusters.previousOrSame( DayOfWeek.SUNDAY ) )  // Specify your desired `DayOfWeek` as start-of-week.
         .atStartOfDay( ZoneId.of( "America/Montreal" ) )

See this code run live at IdeOne.com.

zdt: 2017-07-09T00:00-04:00[America/Montreal] | day-of-week: SUNDAY

Avoid legacy classes

You are using the troublesome old date-time classes that are now legacy, supplanted by the java.time classes.

DayOfWeek

Rather than use mere integer numbers to represent day-of-week in your code, use the DayOfWeek enum built into Java. This gains you type-safety, ensures valid values, and makes your code more self-documenting.

DayOfWeek weekStart = DayOfWeek.SUNDAY ;  // Pass whatever `DayOfWeek` object you want.

TemporalAdjuster & LocalDate

The TemporalAdjuster interface enables ways to manipulate a date to get another date. Find some implementations in TemporalAdjusters class (note plural).

ZoneId z = ZoneId.of( "America/Montreal" ) ;
LocalDate today = LocalDate.now( z ) ;
LocalDate start = today.with( TemporalAdjusters.previousOrSame( weekStart ) ) ;  

ZonedDateTime

To get an exact moment, ask the LocalDate for its first moment of the day. That moment depends on a time zone, as the date varies around the globe for any given moment.

ZonedDateTime zdt = start.atStartOfDay( z ) ;

Instant

If you want to view that some moment as in UTC, extract an Instant object.

Instant instant = zdt.toInstant() ;  // Same moment, different wall-clock time.