Android parse String to Date - unknown pattern character 'X'

No one has mentioned about this error occurring on pre-nougat devices so I thought to share my answer and maybe it is helpful for those who reached this thread because of it.

This answer rightly mentions that "X" is supported only for Nougat+ devices. I still see that documentation suggests to use "yyyy-MM-dd'T'HH:mm:ss.SSSXXX" and not sure why they don't make this point explicit.

For me, yyyy-MM-dd'T'HH:mm:ssXXX was working fine until I tried to test it on 6.0 device and it started crashing which led me to research on this topic. Replacing it with yyyy-MM-dd'T'HH:mm:ss.SSSZZZZZ has resolved the issue and works on all 5.0+ devices.

Remove "XXX" from

DateFormat df = new SimpleDateFormat("yyyy-MM-dd'T'HH:mm:ssXXX");

and everything would work fine.

Go through the list of symbols that can be used inside a SimpleDateFormat constructor. Although the documentation shows the "XXX" format, this doesn't work on Android and will throw an IllegalArgumentException.

Probably you are looking for "yyyy-MM-dd'T'HH:mm:ss.SSSZ"

Change your code to

DateFormat df = new SimpleDateFormat("yyyy-MM-dd'T'HH:mm:ss.SSS"); 

or

DateFormat df = new SimpleDateFormat("yyyy-MM-dd'T'HH:mm:ssZ"); // if timezone is required

The Android version of SimpleDateFormat doesn't support the X pattern so XXX won't work but instead you can use ZZZZZ which does the same and outputs the timezone in format +02:00 (or -02:00 depending on the local timezone).