Struggling with how to compare hours with different time zones in Java?

Question

I have 2 date object in the database that represent the company\'s working hours.

I only need the hours but since I have to save date. it appears like this:

Answer 1

The java.util.Date class is a container that holds a number of milliseconds since 1 January 1970, 00:00:00 UTC. Note that class Date doesn't know anyting about timezones. Use class Calendar if you need to work with timezones. (edit 19-Jan-2017: if you are using Java 8, use the new date and time API in package java.time).

Class Date is not really suited for holding an hour number (for example 13:00 or 18:00) without a date. It's simply not made for that purpose, so if you try to use it like that, as you seem to be doing, you'll run into a number of problems and your solution won't be elegant.

If you forget about using class Date to store the working hours and just use integers, this will be much simpler:

Date userDate = ...;
TimeZone userTimeZone = ...;

int companyWorkStartHour = 13;
int companyWorkEndHour = 18;

Calendar cal = Calendar.getInstance();
cal.setTime(userDate);
cal.setTimeZone(userTimeZone);

int hour = cal.get(Calendar.HOUR_OF_DAY);
boolean withinCompanyHours = (hour >= companyWorkStartHour && hour < companyWorkEndHour);

If you also want to take minutes (not just hours) into account, you could do something like this:

int companyWorkStart = 1300;
int companyWorkEnd = 1830;

int time = cal.get(Calendar.HOUR_OF_DAY) * 100 + cal.get(Calendar.MINUTE);
boolean withinCompanyHours = (time >= companyWorkStart && time < companyWorkEnd);