'It's about time" expresses impatience that something is overdue or late. It can be used on hearing that something has happened that the speaker believes should have happened earlier. It can also be used about something that has not yet happened, but the speaker believes should have already occured. For example I could tell my boss that I have just completed the report that she has been wanting for some time, she could reply "It's about time!" Or, if I have not yet completed the report, she might say "It's about time you did that report!"
'Damn' is an intensifier, and is often seen as mild swearing. It intensifies the expression of impatience