Time Fu Panda is glad you asked.
UTC is Coordinated Universal Time.
UTC does not switch between summer and winter times, but time zones may change their offset from UTC when they make the switch. For example, in the winter, the UK is on GMT (UTC+0), but in the summer they are on BST (UTC+1).
To convert your availabilities to UTC, you can find out the offset of your local timezone and do the maths (there's a nice list here), or you can use a converter, like this one.
A third option is to follow this guide to add a UTC clock to your Windows desktop.