Freezing localtimestamp and other time function on some value

Hello!
Is there any method to freeze localtimestamp and other time function value.
Say after freezing on some value sequential calls to these functions
give you the same value over and over again.
This  is useful primarily for testing.

In oracle there is alter system set fixed_date command. Have Postgres
this functionality?

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Alex Ignatov
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company