We always advice our members to set their boutique's timezone to match the locale they are trading from.
This is especially relevant for time sensitive features such as Coupons which allow you to set a date and time when a coupon code can be used from and until.
To set your boutique's timezone:
Log into your boutique admin area
Click on SHOP SETTINGS >> Shop
Scroll down to the REGIONAL SETTINGS section
Select your timezone from the drop down menu (e.g. "(GMT-08:00) Pacific Time (US & Canada)" if you are in California)
..... and that's it