Why has my Nokia Lumia 520 calendar moved everything 1 day backwards?

[Windows Central Question]

My son & I have the same phones but have never had problems with keeping our calendars separate. Yesterday I suddenly had all his information in my calendar and all my dates were moved 1 day backwards. I went into settings and turned off US Holidays and Birthdays (which were not there before) but all my calendar dates are still 1 day backwards. Please help.....

 

[shmsnh]

Have you checked your calendar time zone settings?