How does the timestamp work in Hummingbird?

You can specify a timezone for all reporting and timestamps within Settings: click on your name on the top right, choose Settings from the drop-down then click on the Organization tab. There's an "Organization Timezone" under the Profile Information section at the top.

If the data you sent us does not have a timestamp, it’s shown in UTC time (approximately London time), which may look a day earlier in the transactions table. The Insights table also appends a time to the record – it shows you precisely where the datetime is being represented on the chart, which is midnight UTC time.