Need Help: Subtract CurrentRowDateTime - PreviousRowDateTime from same column

Here is the JSON for the ETL that you would like to put together. You will have to connect the input dataset to your actual dataset and correct any column name differences, but I tried to match your screenshot.

{"contentType":"domo/dataflow-actions","data":[{"name":"DemoAlarmTime.xlsx","id":"986a7935-4eff-4fe7-878f-7fd7c062f2ce","type":"LoadFromVault","gui":{"x":108,"y":156,"color":3238043},"dependsOn":[],"removeByDefault":false,"notes":[],"dataSourceId":"91adafad-481e-4a95-8403-d11cf696ccbe","executeFlowWhenUpdated":false,"onlyLoadNewVersions":false,"columnSettings":{},"visiblePartitionColumn":null,"versionWindow":null},{"name":"Rank & Window","id":"9e4c6190-59e3-4b51-9466-f096e1386bd8","type":"WindowAction","gui":{"x":228,"y":156},"dependsOn":["986a7935-4eff-4fe7-878f-7fd7c062f2ce"],"removeByDefault":false,"notes":[],"partitionedAggregation":false,"additions":[{"name":"Prev_AlarmTime","operation":{"type":"OFFSET","operationType":"LAG","column":"Alarm Handled Store Time","amount":"1"}}],"orderRules":[{"column":"Alarm ID","caseSensitive":false,"ascending":true}],"groupRules":[{"column":"Tech Name","caseSensitive":false}]},{"name":"Add Formula","id":"96748ab3-23aa-4407-8418-771dba6a6531","type":"ExpressionEvaluator","gui":{"x":348,"y":156},"dependsOn":["9e4c6190-59e3-4b51-9466-f096e1386bd8"],"removeByDefault":false,"notes":[],"expressions":[{"fieldName":"AHT","expression":"TIME_TO_SEC(TIMEDIFF(Alarm Handled Store Time, Prev_AlarmTime)) / 60\n","settings":null}]},{"name":"Filter Rows","id":"45465bd2-2354-4141-8d93-a81c4bd1e79d","type":"Filter","gui":{"x":468,"y":264},"dependsOn":["96748ab3-23aa-4407-8418-771dba6a6531"],"removeByDefault":false,"notes":[],"filterList":[{"leftField":"AHT","rightValue":{"value":"0","type":"DOUBLE"},"operator":"NE","andFilterList":[]}]},{"name":"Remove Duplicates","id":"4f95962d-8672-411a-a9df-4f046dab110f","type":"Unique","gui":{"x":588,"y":264},"dependsOn":["45465bd2-2354-4141-8d93-a81c4bd1e79d"],"removeByDefault":false,"notes":[],"partitionedAggregation":false,"fields":[{"name":"Tech Name","caseInsensitive":false},{"name":"Alarm Handled Store Time","caseInsensitive":false},{"name":"Prev_AlarmTime","caseInsensitive":false}]},{"name":"Join Data","id":"e0cbb736-9ab8-4a23-ac17-8b645b0fe1ad","type":"MergeJoin","gui":{"x":720,"y":156},"dependsOn":["96748ab3-23aa-4407-8418-771dba6a6531","4f95962d-8672-411a-a9df-4f046dab110f"],"removeByDefault":false,"notes":[],"joinType":"LEFT OUTER","step1":"96748ab3-23aa-4407-8418-771dba6a6531","step2":"4f95962d-8672-411a-a9df-4f046dab110f","keys1":["Tech Name","Alarm Handled Store Time"],"keys2":["Tech Name","Alarm Handled Store Time"],"schemaModification1":[],"schemaModification2":[{"name":"Alarm ID","rename":"Remove Duplicates.Alarm ID","remove":true},{"name":"Tech Name","rename":"","remove":true},{"name":"Alarm Handled Store Time","rename":"","remove":true},{"name":"Prev_AlarmTime","rename":"","remove":true},{"name":"AHT","rename":"Prev_AHT","remove":false}],"partitioningInputId":""},{"name":"Add Formula 1","id":"e08ffe31-a541-44c5-8213-e96c568bc691","type":"ExpressionEvaluator","gui":{"x":840,"y":156},"dependsOn":["e0cbb736-9ab8-4a23-ac17-8b645b0fe1ad"],"removeByDefault":false,"notes":[],"expressions":[{"fieldName":"Final_AHT","expression":"CASE when AHT = 0 and Prev_AHT is not null then Prev_AHT else AHT END","settings":null}]},{"name":"DemoAlartmTime","id":"db45f7aa-361e-4526-b80a-de22bd58d60d","type":"PublishToVault","gui":{"x":972,"y":156},"dependsOn":["e08ffe31-a541-44c5-8213-e96c568bc691"],"removeByDefault":false,"notes":[],"dataSource":{"name":"DemoAlartmTime"},"partitionIdColumns":[],"upsertColumns":[],"retainPartitionExpression":""}]}

Here is what it should look like.

You will want to use the Final_AHT field that is calculated at the end.