I have a so easy problem but i cant solve it. I have 3 fields, begintime, and how long service extends, third textfield should be result of these 2 fields.
Like begintime + extendtime = result like 10.00 + 00.30 = 11.30 and so on...all fields are in short time.

There's a strong argument not to store calculated information in a separate field, just perform the calculation when you need to.

But if your two fields are both currently date/time then you can use the function datefdiff() to get a simple difference between the two dates in years, months, days, hours, minutes or seconds.

There's a strong argument not to store calculated information in a separate field,
That is an understatement... Simply do not store calculated values....

But if your two fields are both currently date/time then you can use the function datefdiff() to get a simple difference between the two dates in years, months, days, hours, minutes or seconds.
Adding and datediff??

If the fields are dates, simply add them:
StartTime + Duration

Will return the endtime...