Beware that timing is an issue on Android. Due to power consumption apps normally cannot use an exact timer. Doing so would drain the battery too much.
Thus, inaccurate timers or the system’s alarm manager are used. Both leading to scheduled messages not being sent at the exact desired time.
So, if delayed sending is important to you with correct timestamp, you should not rely on a client running on an Android device.
If your account is an exchange account, you can set the delay on the server through the OWA.