你可以用 java.time 作为比较。。。
有一个 Instant 表示时间上的某个时刻，就像时间戳以毫秒表示一样(⇒ 你的 long nextReminder )以及 OffsetDateTime.now() 现在和现在 LocalDate 仅作为描述日期部分的部分。
你可以看看 nextReminder 今天是用这样的方法：

/**
 * <p>
 * Checks if the day (or date) of a given timestamp (in epoch milliseconds)
 * is the same as <em>today</em> (the day this method is executed).<br>
 * <strong>Requires an offset in order to have a common base for comparison</strong>
 * </p>
 *
 * @param epochMillis   the timestamp in epoch milliseconds to be checked
 * @param zoneOffset    the offset to be used as base of the comparison
 * @return <code>true</code> if the dates of the parameter and today are equal,
 *         otherwise <code>false</code>
 */
public static boolean isToday(long epochMillis, ZoneOffset zoneOffset) {
    // extract the date part from the parameter with respect to the given offset
    LocalDate datePassed = Instant.ofEpochMilli(epochMillis)
                                .atOffset(zoneOffset)
                                .toLocalDate();
    // then extract the date part of "now" with respect to the given offset
    LocalDate today = Instant.now()
                                .atOffset(zoneOffset)
                                .toLocalDate();
    // then return the result of an equality check
    return datePassed.equals(today);
}

然后就这样说吧

boolean isNextReminderToday = isToday(nextReminder, ZoneOffset.systemDefault());

它将使用系统的时间偏移。也许 吧， ZoneOffset.UTC 也可能是个明智的选择。

德哈尔的回答是正确的。但是，我觉得编写这个代码是因为在本例中，使用zone id（而不是zone offset）使代码更简单，也更容易理解。

import java.time.Instant;
import java.time.LocalDate;
import java.time.ZoneId;

public class Main {
    public static void main(String[] args) {
        // A test data
        long nextReminder = 1597754387710L;

        // Your time-zone e.g. Europe/London
        ZoneId zoneId = ZoneId.of("Europe/London");

        // Next reminder date
        Instant instant = Instant.ofEpochMilli(nextReminder);
        LocalDate nextReminderDate = instant.atZone(zoneId).toLocalDate();

        // Today at the time-zone of Europe/London
        LocalDate today = LocalDate.now(zoneId);

        if (today.equals(nextReminderDate)) {
            System.out.println("The next reminder day is today");
        }
    }
}

输出：

The next reminder day is today

使用apache commons DateUtils.isToday(nextReminder) 用你自己的方法。

private static final long MILLIS_PER_DAY = 86400000;

public static boolean isToday(long timestamp) {
   long now =  System.currentTimeMillis();
   long today = now.getTime() / MILLIS_PER_DAY;
   long expectedDay = timestamp / MILLIS_PER_DAY;
   return today == expectedDay;
}

注意：使用日期/时间时，请考虑使用utc。