This weekend Daylight Savings Time will start or end in some parts of the world (Europe, Australia, etc). The Americas will change the clock a week from now. To ensure that the time within Fastmail is synchronized with the timezone of your location, you will need to update your preferences to reflect any change caused by the daylight savings transition. To do this, go to Options -> Account Preferences and select your updated timezone, being either the normal one or the daylight savings one.
For more information on daylight savings and when the transition will take place in your country, please see timeanddate.com.