Hello Mustafa Ahmed,
If you hide, the 18 months will be counted from the date Germany accepted the request of the Netherlands. See the corresponding Article in the Dublin III Regulation, especially part 2:
Modalities and time limits
1. The transfer of the applicant or of another person as referred to in Article 18(1)(c) or (d) from the requesting Member State to the Member State responsible shall be carried out in accordance with the national law of the requesting Member State, after consultation between the Member States concerned, as soon as practically possible, and at the latest within six months of acceptance of the request by another Member State to take charge or to take back the person concerned or of the final decision on an appeal or review where there is a suspensive effect in accordance with Article 27(3).
If transfers to the Member State responsible are carried out by supervised departure or under escort, Member States shall ensure that they are carried out in a humane manner and with full respect for fundamental rights and human dignity.
If necessary, the applicant shall be supplied by the requesting Member State with a laissez passer. The Commission shall, by means of implementing acts, establish the design of the laissez passer. Those implementing acts shall be adopted in accordance with the examination procedure referred to in Article 44(2).
The Member State responsible shall inform the requesting Member State, as appropriate, of the safe arrival of the person concerned or of the fact that he or she did not appear within the set time limit.
2. Where the transfer does not take place within the six months’ time limit, the Member State responsible shall be relieved of its obligations to take charge or to take back the person concerned and responsibility shall then be transferred to the requesting Member State. This time limit may be extended up to a maximum of one year if the transfer could not be carried out due to imprisonment of the person concerned or up to a maximum of eighteen months if the person concerned absconds.
The day of leaving the camp doesn't matter.