Problem Statement -:

We have merged fields with SMS start time field and SMS end time field.

for some reasons.....I guess is due to the one hour difference in time, My event time sent out through sms is showing one hour higher. Example if event  start time is 6 pm and end time 9m, the sms message will be 7pm and 10pm

Solution -:

This seems to be due to Daylight savings time change. We query the date and time field from salesforce database and display the same while resolving merge fields (we don't make any changes explicitly).

When Daylight savings time starts, you will see 1-hour difference because the salesforce automatically adjusts the timezone when Daylight saving starts.

In this case, you need to update right timezone of the user.  Please change the Timezone in Salesforce and let Salesforce handle everything.

Let me know if you need any information further.