Python Desktop Activity Log App
Welcome to CodeReview. Great first question!

Stringly-typed dates

Rather than holding onto dates as strings, you should be holding onto them as actual datetime objects. So this:

# When to start monitoring
start_time = datetime.now().strftime("%H:%M:%S")
# End Time - When to end monitoring
end_time = "08:55:00"


should instead be

# When to start monitoring
start_time = datetime.now()
# End Time - When to end monitoring
# This assumes that we are running early in the morning of the same
# day as the end_time
end_time = datetime.now().replace(hour=8, minute=55)


and

p_start_time &lt;= datetime.now().strftime("%H:%M:%S") &lt;= p_end_time:


should be

p_start_time &lt;= datetime.now() &lt;= p_end_time:


Parametric paths

# Directory - Where to store activity log
directory = r"C:\Users\username\Documents"


should not be hard-coded. Instead, accept it as a command-line argument, or an environmental variable, or store it in a config file.

Path management

# File path / File name
log_path = dir_t + "\\Log.csv"


is much easier with pathlib:

from pathlib import Path
# ...
dir_t = Path(...)
log_path = dir_t / 'Log.csv'


Logging

Strongly consider using the actual logging module instead of rolling your own. This will lend you more flexibility, and access to a more feature-rich logging framework. You can do everything you're doing now - have a file handler; have a custom formatter with your own chosen fields; etc.

In-place addition

        num = num + 1


can be

        num += 1