@TIME[hh:mm:ss[am|pm]] :  Returns the number of seconds since midnight for the specified time. We recommend that you use a 24-hour time format for compatibility with all locales. If "am" or "pm" are specified @TIME will use a 12-hour format. Any non-numeric character, except a right bracket ] can be used to separate the hour, minute and second subfields.

 

Examples:

 

echo %@time[12:34:56]

45296

 

echo %@time[%_time]

81579