You need to use %I (12 hour format) instead of %H (24 hour format). If you do this then there is also no need to set "twelveHourClock" to true. Also, %p is just for adding the 'am/pm'.
From the docs:
If you set "twelveHourClock" to true, the autogenerated timestamps will use 12 hour AM/PM timestamps instead of 24 hour. This only applies if you have not set "timeformat". Use the "%I" and "%p" or "%P" options if you want to build your own format string with 12-hour times.