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All are grammatically correct, and there is little practical difference in their meaning. Noting only the words that change with each example,

"had been up to these days" indicates the actions are in the past, although recent;

"was up to these days" indicatescould indicate the actions are in the present and/or near past;

"has been up to these days" indicates the actions are in the near past but ongoing;past;

"is up to these days" indicates the actions are presently happening and ongoing.

"These days""These days" keeps all of these examples in the present, recent or near past.

All are grammatically correct, and there is little practical difference in their meaning. Noting only the words that change with each example,

"had been up to these days" indicates the actions are in the past, although recent;

"was up to these days" indicates the actions are in the near past;

"has been up to these days" indicates the actions are in the near past but ongoing;

"is up to these days" indicates the actions are presently happening and ongoing.

"These days" keeps all of these examples in the present, recent or near past.

All are grammatically correct, and there is little practical difference in their meaning. Noting only the words that change with each example,

"had been up to these days" indicates the actions are in the past, although recent;

"was up to these days" could indicate the actions are in the present and/or near past;

"has been up to these days" indicates the actions are in the near past;

"is up to these days" indicates the actions are presently happening and ongoing.

"These days" keeps all of these examples in the present, recent or near past.

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All are grammatically correct, and there is little practical difference in their meaning. Noting only the words that change with each example,

"had been up to these days" indicates the actions are in the past, although recent;

"was up to these days" indicates the actions are in the near past;

"has been up to these days" indicates the actions are in the near past but ongoing;

"is up to these days" indicates the actions are presently happening and ongoing.

"These days" keeps all of these examples in the present, recent or near past.