How to add days to date time in Salesforce Apex? - salesforce

Question

Hi I am using Salesforce Apex,
I have a date as String as below. I need to add days to it using Apex.
String dateTime = '2017-07-08T23:59:59Z';
If I add one day to it then it should be 2017-07-09T23:59:59Z as string. How will I do this?
Thanks!

Answer 1

Beware the DST issue! The "addDays" function is not DST-aware, so if you step over a DST transition during the addition of days (in a time zone that has DST) then the time will be messed up.
To resolve this one split the date/time into separate date and time parts first, add the days to the date part then re-combine at the end, like:
DateTime dt = ...;
Integer days = ...;
Date d = dt.date().addDays(days);
Time t = dt.time();
dt = DateTime.newInstance(d, t);
If you are working in the UK (London) time zone the following anonymous Apex illustrates the issue nicely:
DateTime dt = DateTime.newInstance(2017, 10, 28, 23, 59, 59);
System.debug('Adding days directly: ' + dt.addDays(2));
Date d = dt.date().addDays(2);
Time t = dt.time();
dt = DateTime.newInstance(d, t);
System.debug('Adding days in parts: ' + dt);

Answer 2

You need to convert the string to a DateTime and then add days. You can format it back after
String stringDateTime = '2017-07-08T23:59:59Z';
DateTime dt = DateTime.valueOfGmt(stringDateTime);
DateTime tomorrow = dt.addDays(1);
DateTime nextMonth = dt.addMonths(1);
DateTime anniversary = dt.addYears(1);
String formattedDateTime = dt.format('yyyy-MM-dd\'T\'HH:mm:ss\'Z\'');