Short answer: the subscription date and time you see on Leads is drawn in your own device's time zone, not in a time zone stored on the OptiMonk account. If it looks wrong, it is your computer's clock settings that decide it.
Where the date comes from
Open Leads in the left sidebar. Every row ends with a Date column: the day on the first line, the time under it.
OptiMonk stores each submission as one exact moment in time and lets your browser draw it. Your browser takes the time zone from your operating system — so the same lead shows 15:49 for a colleague in Budapest and 09:49 for one in New York, and both are correct.
To see the times in a different zone, change the time zone of the computer (or phone) you are looking at the list on, then reload the page. Nothing has to be changed inside OptiMonk.
The same is true of:
the lead detail panel you get by clicking a row, which repeats the moment as date · time;
the campaign's own Submits tab (Campaigns → your campaign → Submits), which is the same table narrowed to that one campaign.
Filtering by date
The Date range control above the table filters on calendar days. Pick a start and an end day in the calendar and click Apply; Clear removes the filter again. The other filters — Select domain, Select Campaigns, Failed, Columns — can be combined with it.
Because the rows are drawn in your device's zone, a lead that arrived just before midnight in one zone can fall on the neighbouring day in another. If a lead seems to be missing from a range, widen it by a day at each end before anything else.
Exports are a separate thing
Export → Export as XLSX / Export as CSV does not build the file in your browser: OptiMonk generates it and mails it to you — Export started — we'll email it to {email}. That means the timestamps in the file are formatted where the file is made, not by your computer, so they can differ from what the table shows on screen.