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Date
Date d1= Date.today(); //current date
Date d1= Date.newInstance(2022,05,15); //(YYYY,MM,DD)
https://developer.salesforce.com/docs/atlas.en-us.apexref.meta/apexref/apex_methods_system_date.htm
Time
Time t1= Date.newInstance(16,55,30,20); //(HH,MM,SS,MS)
https://developer.salesforce.com/docs/atlas.en-us.apexref.meta/apexref/apex_methods_system_time.htm
Datetime
Datetime dt1 = DateTime.newInstance(2022,05,15,16,55,30); //(YYYY,MM,DD,HH,MM,SS)
https://developer.salesforce.com/docs/atlas.en-us.apexref.meta/apexref/apex_methods_system_datetime.htm