There's no single answer to "What time was it 30 minutes ago?" because the answer changes constantly. To find out what time it was 30 minutes ago, simply look at a clock or watch and subtract 30 minutes.
However, if you're looking for a way to calculate this automatically or programmatically, you would need to use a system clock function within your programming language of choice. Most programming languages have built-in functions to access the current system time and perform calculations like subtracting 30 minutes.
