Please, can anyone tell me how to get current date and time in yang model, i want to apply a must statement in yang which validates that the entered date and time must greater than the current date and time
There is no XPath function to obtain current time in YANG. From RFC 6020, 6.4.1 XPath Context:
The function library is the core function library defined in XPATH, and a function "current()" that returns a node set with the initial context node.
The core function library contains no such function.
Your requirement is a bit odd. Are you sure you would want to constrain a datastore in such a way? If a value were to be modeled this way in a router config, for example, the operator of this router would have to keep updating this config to make it valid.