Intraday charts have to use a timeframe to display data properly. A Timeframe is a set of parameters for the exchange on which the security trades. It includes session start and end times, the rollover time, breaks in trading, if any, days of week when the exchange is active and the exchange holidays. Each security symbol is assigned a timeframe, and the chart takes the timeframe into account when displaying the data.
Medved Trader defines many different timeframes. You can see their parameters on this page.
If, for some reason, you need to define a custom timeframe, the easiest way is to make a copy of an existing one and modify it.