I see several questions there.
1) Convert 25 days into seconds:
| eval 25days = 25 * 86400
2) Convert current time to seconds:
| eval now = now()
3) Display the difference between now and 25 days ago:
| eval diff = now() - relative_time(now(), "-25d")
25days convert to seconds and difference with current time to seconds gives 252460*60 which is 25 days. If you need epoch value of day that is 25 days ago from current time then use relative time.
| makeresults | eval 25daysago_epoch=relative_time(now(), "-25d"), 25daysago_datetime=strftime('25daysago_epoch', "%d-%m-%Y %H:%M:%S"), current_time=strftime(_time, "%d-%m-%Y %H:%M:%S") | table current_time, 25daysago_datetime, 25daysago_epoch