Hi @Tejaswini1234 ,
Could you please share a bit more about your scenario?
Do you want to calculate the difference between two date column values using Microsoft Flow?
Currently, within Microsoft Flow, there is no direct way to calculate the difference between two different date values.
As an alternative solution, I think the ticks() function within Microsoft Flow WDL expression could achieve your needs. I have made a test on my side, please take a try with the following workaround:

Add a "Initialize variable" action to initialize a variable to store the date difference.
Within the "StartTimeTickValue" action ("Compose" action), Inputs field set to following:
ticks(triggerBody()?['StartTime'])
Within the "EndTimeTickValue" action ("Compose" action), Inputs field set to following:
ticks(triggerBody()?['EndTime'])
Within the "DifferenceAsMinutes" action ("Compose" action), Inputs field set to following:
div(div(mul(sub(outputs('EndTimeTickValue'),outputs('StartTimeTickValue')),100),1000000000),60)
Within the "Set variable" action, Value field set to Output dynamic content of the above "DifferenceAsMinutes" action.
The "StartTime" and "EndTime" are both date time type columns in my SP list. The passed "StartTime" value and "EndTime" value as below:
The flow executes successfully as below:
If you want to calculate the Hours difference between two different date values, please consider go to the "DifferenceAsMinutes" action, then modify the Inputs field value to following:
div(div(mul(sub(outputs('EndTimeTickValue'),outputs('StartTimeTickValue')),100),1000000000), 3600)
If you want to calculate the Hours difference between two different date values, please modify above formula as below:
div(div(mul(sub(outputs('EndTimeTickValue'),outputs('StartTimeTickValue')),100),1000000000), 86400)
More details about using expression in flow actions, please check the following blog:
https://flow.microsoft.com/en-us/blog/use-expressions-in-actions/
In addition, you could also consider achieve your needs using the DateDiff() function within PowerApps. More details about the DateDiff() function in PowerApps, please check the following article:
DateDiff function
Best regards,