Rest day/period
Regarding Flight Time Limitations, your bible is called CAA-CAP 371 ( UK ).
Most operators around the world base their scheme on the CAP371, except those "engaged in air taxi/sole-use charter, air ambulance work, and pleasure flying".
It is said there, Section B-17.1,2,...,6:
" The aircraft operator must notify all crew members in good time of a flying duty period so that sufficient and uninterrupted pre-flight rest can be obtained. When away from base the operator must provide the crew suitable accomodation. When flights are carried out at such short notice that it is impracticable for an operator to arrange suitable accomodation, then this responsability devolves to the aircraft commander."
" The minimum rest period which must be provided before undertaking a flying duty period shall be:
_ at least as long as the preceding duty period or,
_12 hours
whichever is the greater. "
" Exceptionally at home base, individual crew members may be asked to exercise their discretion to reduce rest by up to a minimum of one hour but only to a minimum of 12 hours for flight crew and 11 hours for cabin crew. "
" If the preceding duty period, which includes any time spent on positioning, exceeded 18 hours, then the ensuing rest period must include a local night. "
" After being called out from a standby duty the length of minimum rest shall be determined by the length of standby duty, plus any time spent on positioning, and any FDP completed. "
" Crew members who inform an operator tha they are having difficulty in achieving adequate pre-flight rest must be given the opportunity to consult an aviation medical specialist. "
Etc, etc,etc ...
Now if you open for example the EK FOM relating to the FTL scheme, Chap. 21, you will find exactly the same definitions and rules...
So you can still download CAP371 from the CAA's website, but to make it simple, let's say that operators, when they don't need to stricly apply these rules, will considere a "legal" Rest day as a 24h rest period ( from 00h01 to 23h59 ), before an " Early Start Duty " or " Night Duty " starting the following day.
I believe that your company can decide to change your incoming duty during that rest period, however they must provide you with a new minimum rest period ( defined by the length of your previous duty or must be at least 12 hours but can be reduced to 11 hours, blablabla... ).
It is also said that they can only contact you during the " contactable period ", which is " a short period of time during the day, other than a day off, during which the company requires a crew member to be at an agreed location for the purpose of giving notification of a duty period which will commence not less than 10 hours ahead." Also this contactable period does not constitute a stand-by duty.
Anyway, we could go on for hours like that...
Now you have understand that you always have 2 options: let them screw you, or play with the legalities' fire!
Enjoy...