Jodatime start of day and end of day

also a simple way is

d.millisOfDay().withMaximumValue();

You can use the withTime method:

 d.withTime(0, 0, 0, 0);
 d.withTime(23, 59, 59, 999);

Same as Peter's answer, but shorter.

How about:

private LocalDateTime calcNextSunday(LocalDateTime d) {
    return d.withHourOfDay(23).withMinuteOfHour(59).withSecondOfMinute(59).withDayOfWeek(DateTimeConstants.SUNDAY);
}

private LocalDateTime calcPreviousMonday(final LocalDateTime d) {
    return d.withHourOfDay(0).withMinuteOfHour(0).withSecondOfMinute(0).withDayOfWeek(DateTimeConstants.MONDAY);
}

For those coming here looking for the answer for "js-joda", you have two options depending on what you're looking to accomplish

Option 1: You want the start of the day in the same timezone

Since you've chosen to calculate your times based on an instant in time in relation to a timezone, you should use ZonedDateTime:

import { ZonedDateTime, LocalDate, ZoneId, DateTimeFormatter} from "js-joda";
import 'js-joda-timezone';

const nowInNewYorkCity = ZonedDateTime.now(ZoneId.of("America/New_York"))
const startOfTodayInNYC = nowInNewYorkCity.truncatedTo(ChronoUnit.DAYS);
console.log(startOfTodayInNYC.toString()) // Prints "2019-04-15T00:00-04:00[America/New_York]"
// And if you want to print it in ISO format
console.log(startOfTodayInNYC.format(DateTimeFormatter.ISO_INSTANT)) // "2019-04-14T04:00:00Z"

Option 2: You know the exact day that you want to get the time for

Then you can use the following methods off of LocalDate to derive the relative time (i.e. ZonedDateTime) you'd like:

    atStartOfDay(): LocalDateTime
    atStartOfDay(zone: ZoneId): ZonedDateTime
    atStartOfDayWithZone(zone: ZoneId): ZonedDateTime

Option 3: I want just the day the instant occurred on

Notice with this code, you get the day that it would be relative to where you are. So for those in New York City, it's "2019-04-14" and for those in London it would be "2019-04-15" (which is great!) because the instant in time was during the period of time where it's actually tomorrow in London ("2019-04-15T00:00:05Z"). Pretend that you were calling someone in London from NYC, and the Londoner would say, "geez, why are you calling me so early... it's 5 seconds past midnight."

import { ZonedDateTime, LocalDate, ZoneId} from "js-joda";
import 'js-joda-timezone';

const aTimeWhenLondonIsAlreadyInTomorrow = "2019-04-15T00:00:05.000Z";
const inBetweenTimeInLondon = ZonedDateTime.parse(aTimeWhenLondonIsAlreadyInTomorrow);
const inBetweenTimeInNYC = inBetweenTimeInLondon.withZoneSameInstant(ZoneId.of("America/New_York"))
const dayInLondon = inBetweenTimeInLondon.toLocalDate();
const dayInNYC = inBetweenTimeInNYC.toLocalDate();
console.log(inBetweenTimeInLondon.toString()); // "2019-04-15T00:00:05Z"
console.log(dayInLondon.toString()); // "2019-04-15"
console.log(inBetweenTimeInNYC.toString()) // "2019-04-14T20:00:05-04:00[America/New_York]"
console.log(dayInNYC.toString()); // "2019-04-14"

References: https://js-joda.github.io/js-joda/class/src/LocalDate.js~LocalDate.html#instance-method-atStartOfDayWithZone