Android app for logging screentime sessions

I am searching for an Android app that logs start and end point of each usage, not just the length. I do not need details about the used application, just time and date of when the phone screen was switched on will be enough.

The log should contain information such as:

2014-07-13 10:30 switched on
2014-07-13 10:35 switched off
2014-07-13 12:00 switched on
2014-07-13 12:20 switched off

Other requirements:

• Ability to export the data
• Unobtrusive, should not interfere with my normal usage.
• Should require minimal permissions, preferably no root and no internet access
• Free of advertisements is strongly preferred.
• A reasonable price is fine