SSAS Tabular model Date Dimension with Time possible?
Generally I keep Date and Time in separate dimensions as this reduces the overall cardinality but even with your date dimension having a time element, you should not have a problem obtaining this value. The example below was written on my own system and tested in DAX Studio: 

DEFINE 
    MEASURE Sales[TransactionCountPerTimePeriod] = COUNTROWS('Sales')
EVALUATE
SUMMARIZECOLUMNS (
    'Date'[DateKey],
    "Transaction Count", [TransactionCountPerTimePeriod]
);


In order to get this work within a model, just take the definition of the measure and include it, like so:

TransactionCountPerTimePeriod:=COUNTROWS('Sales')


Then, after publishing your model you should see it show up in DAX Studio, Power BI, Excel or any other reporting tool:

-- no need to define the measure if it is already in the model
-- just use it
EVALUATE
SUMMARIZECOLUMNS (
    'Date'[DateKey],
    "Transaction Count", [TransactionCountPerTimePeriod]
);