When does the calendar interval for the next completion of a maintenance task start if an extension is made?

See EASA FAQ n. 19102.

If the TCH includes in its ICAs instructions for the calculation of the next completion, these should be followed. 

If not, the date of the next completion will be the expiry date prior to the extension or as agreed with the authority, the final decision must be made by the extension approver. 

For Part-ML regulated aircraft, the situation is different when the 1 month tolerance in ML.A.302(d) applies, the following interval will be calculated from the date of compliance (see ML.A.302(d)(1) and AMC1 ML.A.302(d)).